IB/core: Fix memory corruption in ib_cache_gid_set_default_gid

When ib_cache_gid_set_default_gid is called from several threads,
updating the table could make find_gid fail, therefore a negative
index will be retruned and an invalid table entry will be used.
Locking find_gid as well fixes this problem.

Fixes: 03db3a2d ('IB/core: Add RoCE GID table management')
Signed-off-by: Doron Tsur <doront@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>