switch open-coded instances of d_make_root() to new helper

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 3ce97b2..81df3fe 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -629,7 +629,6 @@
 			    void *data, int silent)
 {
 	struct inode *inode;
-	struct dentry *root_dentry;
 	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
 	struct btrfs_key key;
 	int err;
@@ -660,15 +659,12 @@
 		goto fail_close;
 	}
 
-	root_dentry = d_alloc_root(inode);
-	if (!root_dentry) {
-		iput(inode);
+	sb->s_root = d_make_root(inode);
+	if (!sb->s_root) {
 		err = -ENOMEM;
 		goto fail_close;
 	}
 
-	sb->s_root = root_dentry;
-
 	save_mount_options(sb, data);
 	cleancache_init_fs(sb);
 	sb->s_flags |= MS_ACTIVE;