IB/core: don't search the GID table twice

Previously, we've searched the GID table twice: first when we searched
the table for a GID matching the proposed new one, and second when we
didn't find a match, we searched again for an empty GID slot in the
table.  Instead, search the table once noting the first empty slot as
we search for our target GID.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>