btrfs: use iget5_locked_rcu

With 20 threads each walking a dedicated 1000 dirs * 1000 files
directory tree to stat(2) on a 32 core + 24GB ram vm:

before: 3.54s user 892.30s system 1966% cpu 45.549 total
after:  3.28s user 738.66s system 1955% cpu 37.932 total (-16.7%)

Benchmark can be found here: https://people.freebsd.org/~mjg/fstree.tgz

Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: Mateusz Guzik <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Acked-by: David Sterba <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: mjguzik

fs/btrfs/inode.c

@@ -5587,7 +5587,7 @@ static struct inode *btrfs_iget_locked(struct super_block *s, u64 ino,
 	args.ino = ino;
 	args.root = root;
 
-	inode = iget5_locked(s, hashval, btrfs_find_actor,
+	inode = iget5_locked_rcu(s, hashval, btrfs_find_actor,
 			     btrfs_init_locked_inode,
 			     (void *)&args);
 	return inode;