Suppress AIO kmem warnings

The new zpl_aio_write() and zpl_aio_read() functions use kmem_alloc()
to allocate enough memory to hold the vectorized IO.  While this
allocation will be small it's been observed in practice to sometimes
slightly exceed the 8K warning threshold by a few kilobytes.
Therefore, the KM_NODEBUG flag has been added to suppress warning.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #2774
Illumos 4924 - LZ4 Compression for metadata

Reviewed by Matthew Ahrens <mahrens@delphix.com>
Reviewed by Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:
  https://github.com/illumos/illumos-gate/commit/b8289d2
  https://www.illumos.org/issues/3756

Porting notes:

The static function zfs_prop_activate_feature() was removed because
this change removes the only caller.  The function was not removed
from Illumos but instead left as dead code.  However, to keep gcc
happy it was removed from Linux and may be easily restored if needed.

Ported by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1540