IB/core: Change per-entry lock in RoCE GID table to one lock

Previously, IB GID cached used a lock per entry. This could result
in spending a lot of CPU cycles for locking and unlocking just
in order to find a GID. Changing this in favor of one lock per
a GID table.
Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>