Illumos 3749 - zfs event processing should work on R/O root filesystems

3749 zfs event processing should work on R/O root filesystems
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Christopher Siden <christopher.siden@delphix.com>

References:

https://www.illumos.org/issues/3749
https://github.com/illumos/illumos-gate/commit/3cb69f7

Porting notes:

• [include/sys/spa_impl.h]
  • ffe9d38 Add generic errata infrastructure
  • 1421c89 Add visibility in to arc_read
• [include/sys/fm/fs/zfs.h]
  • 2668527 Add linux events
  • 6283f55 Support custom build directories and move includes
• [module/zfs/spa_config.c]
  • Updated spa_config_sync() to match illumos with the exception of a Linux specific block.

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>