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