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: 03db3a2d81e6 ('IB/core: Add RoCE GID table management')
Signed-off-by: Doron Tsur <[e-mail]>
Signed-off-by: Matan Barak <[e-mail]>
Signed-off-by: Doug Ledford <[e-mail]>

drivers/infiniband/core/cache.c

···
 	memset(&gid_attr, 0, sizeof(gid_attr));
 	gid_attr.ndev = ndev;
 
+	mutex_lock(&table->lock);
 	ix = find_gid(table, NULL, NULL, true, GID_ATTR_FIND_MASK_DEFAULT);
 
 	/* Coudn't find default GID location */
 	WARN_ON(ix < 0);
 
-	mutex_lock(&table->lock);
 	if (!__ib_cache_gid_get(ib_dev, port, ix,
 				&current_gid, &current_gid_attr) &&
 	    mode == IB_CACHE_GID_DEFAULT_MODE_SET &&