Diff reduction against FreeBSD.  This also solves the hang issue when
detaching device from mirror.

Ticket: #4037
Improve ZFS N-way mirror read performance by using load and locality
information.

The existing algorithm selects a preferred leaf vdev based on offset of the zio
request modulo the number of members in the mirror. It assumes the devices are
of equal performance and that spreading the requests randomly over both drives
will be sufficient to saturate them. In practice this results in the leaf vdevs
being under utilized.

The new algorithm takes into the following additional factors:
* Load of the vdevs (number outstanding I/O requests)
* The locality of last queued I/O vs the new I/O request.

Within the locality calculation additional knowledge about the underlying vdev
is considered such as; is the device backing the vdev a rotating media device.

This results in performance increases across the board as well as significant
increases for predominantly streaming loads and for configurations which don't
have evenly performing devices.

The following are results from a setup with 3 Way Mirror with 2 x HD's and
1 x SSD from a basic test running multiple parrallel dd's.

With pre-fetch disabled (vfs.zfs.prefetch_disable=1):

== Stripe Balanced (default) ==
Read 15360MB using bs: 1048576, readers: 3, took 161 seconds @ 95 MB/s
== Load Balanced (zfslinux) ==
Read 15360MB using bs: 1048576, readers: 3, took 297 seconds @ 51 MB/s
== Load Balanced (locality freebsd) ==
Read 15360MB using bs: 1048576, readers: 3, took 54 seconds @ 284 MB/s

With pre-fetch enabled (vfs.zfs.prefetch_disable=0):

== Stripe Balanced (default) ==
Read 15360MB using bs: 1048576, readers: 3, took 91 seconds @ 168 MB/s
== Load Balanced (zfslinux) ==
Read 15360MB using bs: 1048576, readers: 3, took 108 seconds @ 142 MB/s
== Load Balanced (locality freebsd) ==
Read 15360MB using bs: 1048576, readers: 3, took 48 seconds @ 320 MB/s

In addition to the performance changes the code was also restructured, with
the help of Justin Gibbs, to provide a more logical flow which also ensures
vdevs loads are only calculated from the set of valid candidates.

The following additional sysctls where added to allow the administrator
to tune the behaviour of the load algorithm:
* vfs.zfs.vdev.mirror.rotating_inc
* vfs.zfs.vdev.mirror.rotating_seek_inc
* vfs.zfs.vdev.mirror.rotating_seek_offset
* vfs.zfs.vdev.mirror.non_rotating_inc
* vfs.zfs.vdev.mirror.non_rotating_seek_inc

These changes where based on work started by the zfsonlinux developers:
https://github.com/zfsonlinux/zfs/pull/1487

Reviewed by:	gibbs, mav, will
MFC after:	2 weeks
Sponsored by:	Multiplay

(cherry picked from commit 5c7a6f5d929fa744b3a959b187d32c752a5ea7d5)
fix a bug in ZFS mirror code for handling multiple DVAa

The bug was introduced in r256956 "Improve ZFS N-way mirror read
performance".
The code in vdev_mirror_dva_select erroneously considers already
tried DVAs for the next attempt.  Thus, it is possible that a failing DVA
would be retried forever.
As a secondary effect, if the attempts fail with checksum error, then
checksum error reports are accumulated until the original request
ultimately fails or succeeds.  But because retrying is going on indefinitely
the cheksum reports accumulation will effectively be a memory leak.

Reviewed by:	gibbs
MFC after:	13 days
Sponsored by:	HybridCluster

(cherry picked from commit e186f564bc946f82c76e0b34c2f0370ed9aea022)
Restore ABI compatibility.

No flag is required for VERSION_04 because the way the GEOM code supports
rotation rate.
Redo r224921:

Subscribe to attribute change notifications and update vdev
physical path information (in core and on disk) when a
GEOM::physpath event indicates they have changed.
zfs_deleteextattr: name buffer from namei is needed by zfs_rename

If we prematurely free the name buffer and it gets quickly recycled,
then zfs_rename may see data from another lookup or even unmapped memory
via cn_nameptr.

MFC after:	6 days
Sponsored by:	HybridCluster

(cherry picked from commit 113f9a4f5312f631ee8d97ab8641c3064e6a60ca)
zfs: getnewvnode_reserve must be called outside of a zfs transaction

Otherwise we could run into the following deadlock.
A thread has a transaction open and assigned to a transaction group.
That would prevent the transaction group from be quiesced and synced.
The thread is blocked in getnewvnode_reserve waiting for a vnode to
a be reclaimed.  vnlru thread is blocked trying to enter ZFS VOP because
a filesystem is suspended by an ongoing rollback or receive operation.
In its turn the operation is waiting for the current transaction group
to be synced.

zfs_zget is always used outside of active transactions, but zfs_mknode
is always used in a transaction context.  Thus, we hoist
getnewvnode_reserve from zfs_mknode to its callers.

While there, assert that ZFS always calls getnewvnode while having
a vnode reserved.

Reported by:	adrian
Tested by:	adrian
MFC after:	17 days
Sponsored by:	HybridCluster

(cherry picked from commit 97986ccb0b599062a18e0153ae8cba713fde7df5)
fix a build problem with INVARIANTS enabled introduced in r260704

Reported by:	glebius
MFC after:	5 days
X-MFC with:	r260704

(cherry picked from commit d1073990171ca168996fd569bd5dc65018f836bb)
zinject must use ioctl(2) compatibility wrapper

MFC after:	8 days
Sponsored by:	HybridCluster

(cherry picked from commit f9bc6dc89c596d9c742b7f12fbbaad8656b640fe)
zdb -R: do not treat numeric parameters to a flag as more flags

Reviewed by:	Matthew Ahrens <mahrens@delphix.com>
MFC after:	1 week

(cherry picked from commit e1e8b09fdeff5e07793910cdc98e6880b323332e)
traverse_visitbp: visit DMU_GROUPUSED_OBJECT before DMU_USERUSED_OBJECT

This is done to ensure that visited object IDs are always increasing.
Also, pass correct object ID to prefetch_dnode_metadata for
os_groupused_dnode.

Without this change we would hit an assert if traversal was paused on
a GROUPUSED object, which is unlikely but possible.

Apparently the same change was independently developed by Deplhix.

Reviewed by:	Matthew Ahrens <mahrens@delphix.com>
MFC after:	10 days
Sponsored by:	HybridCluster

(cherry picked from commit 6b143ee35a90b366e891df1adc9e3be518352cb9)