IB/core: Ignore membership bit in ib_find_pkey()

ib_find_pkey() is used as a replacement for ib_find_cached_pkey(), and
the original function ignored the membership bit when searching for a
P_Key, so ib_find_pkey() should ignore the bit too.

In particular, IPoIB turns on the P_Key membership bit of limited
membership P_Keys when creating a child interface and looks for the
full membership P_key.  This broke if a port was a partial member of a
partition when IPoIB switched from ib_find_cached_pkey() to
ib_find_pkey(), and this change fixes things again.

Signed-off-by: Moni Shoua <monis@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>