btrfs: use btrfs_tree_read_lock in btrfs_search_slot

We no longer use recursion, so
__btrfs_tree_read_lock(BTRFS_NESTING_NORMAL) == btrfs_tree_read_lock.
Replace this call with the simple helper.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -2802,7 +2802,7 @@ cow_done:
 				btrfs_tree_lock(b);
 				p->locks[level] = BTRFS_WRITE_LOCK;
 			} else {
-				__btrfs_tree_read_lock(b, BTRFS_NESTING_NORMAL, 0);
+				btrfs_tree_read_lock(b);
 				p->locks[level] = BTRFS_READ_LOCK;
 			}
 			p->nodes[level] = b;