Illumos #2882, #2883, #2900

2882 implement libzfs_core
2883 changing "canmount" property to "on" should not always remount dataset
2900 "zfs snapshot" should be able to create multiple, arbitrary snapshots at once

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Chris Siden <christopher.siden@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Bill Pijewski <wdp@joyent.com>
Reviewed by: Dan Kruchinin <dan.kruchinin@gmail.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:
  illumos/illumos-gate@4445fffbbb1ea25fd0e9ea68b9380dd7a6709025

Ported-by: Tim Chase <tim@chase2k.com>
Closes #1293

Porting notes:

Add "nosleep" versions of the fnvlist_dup, fnvlist_merge, fnvlist_alloc
and fnvlist_pack functions with a suffix of "_nosleep".  They simply
perform their allocations with the KM_NOSLEEP flag.

Add zvol minor device creation to the new zfs_snapshot_nvl function.

Remove the logging of the "release" operation in
dsl_dataset_user_release_sync().  The logging caused a null dereference
because ds->ds_dir is zeroed in dsl_dataset_destroy_sync() and the
logging functions try to get the ds name via the dsl_dataset_name()
function. I've got no idea why this particular code would have worked
in Illumos.  This code has subsequently been completely reworked in
Illumos commit 3b2aab1 (3464 zfs synctask code needs restructuring).

Squash some "may be used uninitialized" warning/erorrs.

Fix some printf format warnings for %lld and %llu.

Apply a few spa_writeable() changes that were made to Illumos in
illumos/illumos-gate.git@cd1c8b8 as part of the 3112, 3113, 3114 and
3115 fixes.

Add a missing call to fnvlist_free(nvl) in log_internal() that was added
in Illumos to fix issue 3085 but couldn't be ported to ZoL at the time
(zfsonlinux/zfs@9e11c73) because it depended on future work.

Conflicts:
	include/sys/dmu_objset.h
	module/zfs/dmu_objset.c
Revert #3464 changes that crept into #2882 commit

Signed-off-by: Richard Yao <ryao@gentoo.org>
Illumos #3098

3098 zfs userspace/groupspace fail without saying why when run as non-root

Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

Ported-by: Tim Chase <tim@chase2k.com>

Conflicts:
	lib/libzfs/libzfs_dataset.c
Illumos #3236

3236 zio nop-write

Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

References:
  illumos/illumos-gate@80901aea8e78a2c20751f61f01bebd1d5b5c2ba5
  https://www.illumos.org/issues/3236

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

Conflicts:
	module/zfs/dmu.c
	module/zfs/spa_misc.c
Constify first argument of dmu_objset_find()

This enables us to avoid typecasting (char *) in other areas of the
code. In particular, the zvol code.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Import zvol_create_minors() from FreeBSD

The current zvol_create_minors() code depends on dmu_objset_find_spa(),
which is removed as of Illumos #3464. We can use FreeBSD's function as a
replacement with a few changes.

1. printf() becomes printk()

2. zvol_inhibit_dev support is checked in zvol_create_minors()

3. The helper function zvol_create_snapshots is altered to stop printing
a warning when it sees ENODEV from zvol_create_minor(). That means that
snapdev=hidden is set.

Ported-by: Richard Yao <ryao@gentoo.org>
Illumos 3464 zfs synctask code needs restructuring

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Ported-by: Richard Yao <ryao@gentoo.org>
25068 arc_evict_ghost() should not hold arcs_mtx for a long time

glibc provides sched_yield() rather than yield(). kpreempt() in
zfs_context.h was adjusted to reflect this.

Ported-by: Richard Yao <ryao@gentoo.org>
21470 dsl_dataset_t ds_dir->dd_lock is highly contended
25529 assertion failed in dsl_dataset_check_quota()

Ported-by: Richard Yao <ryao@gentoo.org>