target: Check number of unmap descriptors against our limit

commit 7409a6657aebf8be74c21d0eded80709b27275cb upstream.

Fail UNMAP commands that have more than our reported limit on unmap
descriptors.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>