Btrfs: Various small fixes.

This trivial patch contains two locking fixes and a off by one fix.

---

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index a6a418b..eb4b357 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -449,7 +449,9 @@
 		s->s_flags |= MS_ACTIVE;
 	}
 
+	mutex_lock(&s->s_root->d_inode->i_mutex);
 	root = lookup_one_len(subvol_name, s->s_root, strlen(subvol_name));
+	mutex_unlock(&s->s_root->d_inode->i_mutex);
 	if (IS_ERR(root)) {
 		up_write(&s->s_umount);
 		deactivate_super(s);