Add txgs-<pool> kstat file

Create a kstat file which contains useful statistics about the
last N txgs processed.  This can be helpful when analyzing pool
performance.  The new KSTAT_TYPE_TXG type was added for this
purpose and it tracks the following statistics per-txg.

  txg          - Unique txg number
  state        - State (O)pen/(Q)uiescing/(S)yncing/(C)ommitted
  birth;       - Creation time
  nread        - Bytes read
  nwritten;    - Bytes written
  reads        - IOPs read
  writes       - IOPs write
  open_time;   - Length in nanoseconds the txg was open
  quiesce_time - Length in nanoseconds the txg was quiescing
  sync_time;   - Length in nanoseconds the txg was syncing

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Log I/Os longer than zio_delay_max (30s default)

There have been reports of ZFS deadlocking due to what appears to
be a lost IO.  This patch addes some debugging to determine the
exact state of the IO which neither 1) completed, 2) failed, or
3) timed out after zio_delay_max (30) seconds.

This information will be logged using the ZFS FMA infrastructure
as a 'delay' event and posted to the internal zevent log.  By
default the last 64 events will be kept in the log but the limit
is configurable via the zfs_zevent_len_max module option.

To dump the contents of the log use the 'zpool events -v' command
and look for the resource.fs.zfs.delay event.  It will include
various information about the pool, vdev, and zio which may shed
some light on the issue.

In the context of this change the 120 second kernel blocked thread
watchdog has been disabled for synchronous IOs.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #930
Merge remote branch 'eris/stats'

Bring in support for the new KSTAT_TYPE_TXG type.  This allows for
additional visibility in to the txg handling.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Fix zvol+btrfs hang

When using a zvol to back a btrfs filesystem the btrfs mount
would hang.  This was due to the bio completion callback used
in btrfs assuming that lower level drivers would never modify
the bio->bi_io_vecs after they were submitted via bio_submit().
If they are modified btrfs will miscalculate which pages need
to be unlocked resulting in a hang.

It's worth mentioning that other file systems such as ext[234]
and xfs work fine because they do not make the same assumption
in the bio completion callback.

The most straight forward way to fix the issue is to present
the semantics expected by btrfs.  This is done by cloning the
bios attached to each request and then using the clones bvecs
to perform the required accounting.  The clones are freed after
each read/write and the original unmodified bios are linked back
in to the request.

Signed-off-by: Chris Wedgwood <cw@f00f.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #469
Fix "allocating allocated segment" panic

Gunnar Beutner did all the hard work on this one by correctly
identifying that this issue is a race between dmu_sync() and
dbuf_dirty().

Now in all cases the caller is responsible for preventing this
race by making sure the zfs_range_lock() is held when dirtying
a buffer which may be referenced in a log record.  The mmap
case which relies on zfs_putpage() was not taking the range
lock.  This code was accidentally dropped when the function
was rewritten for the Linux VFS.

This patch adds the required range locking to zfs_putpage().

It also adds the missing ZFS_ENTER()/ZFS_EXIT() macros which
aren't strictly required due to the VFS holding a reference.
However, this makes the code more consistent with the upsteam
code and there's no harm in being extra careful here.

Original-patch-by: Gunnar Beutner <gunnar@beutner.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #541