IB/sa_query: Check if sm_ah is NULL in ib_sa_remove_one()

If update_sm_ah() fails, it leaves the port's sm_ah as NULL.  Then if
the device or module is removed, ib_sa_remove_one() will dereference a
NULL pointer when it calls kref_put().  Fix this by testing if sm_ah
is NULL before dropping the reference.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>