Btrfs: fix subvol_name leak on error in btrfs_mount()

btrfs_parse_early_options() can fail due to error while scanning devices
(-o device= option), but still strdup() subvol_name string:

mount -o subvol=SUBV,device=BAD_DEVICE <dev> <mnt>

So free subvol_name string on error.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/btrfs/super.c

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6befcaf..58e9492 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -905,8 +905,10 @@
 	error = btrfs_parse_early_options(data, mode, fs_type,
 					  &subvol_name, &subvol_objectid,
 					  &subvol_rootid, &fs_devices);
-	if (error)
+	if (error) {
+		kfree(subvol_name);
 		return ERR_PTR(error);
+	}
 
 	if (subvol_name) {
 		root = mount_subvol(subvol_name, flags, device_name, data);