MFC r272507: MFV r272496:

Add tunable for number of metaslabs per vdev
(vfs.zfs.vdev.metaslabs_per_vdev).  The default remains
at 200.

Illumos issue:
    5161 add tunable for number of metaslabs per vdev
MFC r272510: MFV r272498:

Add a new sysctl, vfs.zfs.vol.unmap_enabled, which allows the system
administrator to toggle whether ZFS should ignore UNMAP requests.

Illumos issue:
    5149 zvols need a way to ignore DKIOCFREE
MFC r272511: MFV r272499:

Illumos issue:
    5174 add sdt probe for blocked read in dbuf_read()
MFC r272584: MFV r272501:

Illumos issue:
    5177 remove dead code from dsl_scan.c
MFC r272598: MFV r272585:

Split the godfather zio into CPU number's to reduce lock
contention.

Illumos issue:
    5176 lock contention on godfather zio
MFC r272599: MFV r272588:

Handle old format deadlist.

Illumos issue:
    5178 zdb -vvvvv on old-format pool fails in dump_deadlist()