Add BIO_DELETE support to ZVOL.

It is an adapted merge from the vendor branch of:
    701 UNMAP support for COMSTAR  (in part related to ZFS)
    2130 zvol DKIOCFREE uses nested DMU transactions

(cherry picked from commit cd80e1706c6a79b53f1c4da94d86f0b49098434d)

Conflicts:
	sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zvol.c
In addition to r264077, tell GEOM that we do support BIO_DELETE now.

(cherry picked from commit 2e86f49ec8904342444286fc06d9b46ed245052c)