[PATCH] IB: Fix user MAD registrations with class 0

Fix handling of MAD agent registrations with mgmt_class == 0.  In this case
ib_umad should pass a NULL registration request to the MAD core rather than a
request with mgmt_class set to 0.

Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>