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 after:	2 weeks
Add in decode for the AR9300 RX descriptor.
Diff reduction with upstream.  The code change is not really applicable
to FreeBSD.

Illumos issue:
    5148 zvol's DKIOCFREE holds zfsdev_state_lock too long

MFC after:	1 month