Merge the ZFS I/O deadman thread from vendor (illumos).
This feature panics the system on hanging ZFS I/O, helps debugging
and resumes failed service.
The panic behavior can be controlled with the loader-only tunables:
vfs.zfs.deadman_enabled (enable or disable panic on stalled ZFS I/O)
vfs.zfs.deadman_synctime (expiration time for stalled ZFS I/O)
By default, ZFS I/O deadman is enabled by default on amd64 and i386
excluding virtual guest machines.
Illumos ZFS issues:
3246 ZFS I/O deadman thread
References:
https://www.illumos.org/issues/3246
MFC after: 2 weeks
e70664bafc