btrfs: remove a pointless NULL check in btrfs_lookup_fs_root

btrfs_grab_root already checks for a NULL root itself.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -1157,8 +1157,7 @@ static struct btrfs_root *btrfs_lookup_fs_root(struct btrfs_fs_info *fs_info,
 	spin_lock(&fs_info->fs_roots_radix_lock);
 	root = radix_tree_lookup(&fs_info->fs_roots_radix,
 				 (unsigned long)root_id);
-	if (root)
-		root = btrfs_grab_root(root);
+	root = btrfs_grab_root(root);
 	spin_unlock(&fs_info->fs_roots_radix_lock);
 	return root;
 }