[PATCH] IB: allow NULL sa_query callbacks

Check if a client passes a NULL callback into an SA query, and if so, never
call back.  This fixes an oops if someone unloads ib_ipoib and ib_sa in
rapid succession.  ib_ipoib does an MCMember delete with a NULL callback
and 0 timeout on unload, which is usually fine since the delete completes
successfully.  However, if ib_sa is unloaded immediately afterwards, the
delete will be canceled and ib_sa will try to call the (now already
unloaded) ib_ipoib module back with the cancel completion, which triggers
the oops.

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