ext4: fix missing ext4_unlock_group in error path

If we try to free a block which is already freed, the code was
returning without first unlocking the group.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index dfe17a1..444ad99 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -4441,6 +4441,7 @@
 		else if (block >= (entry->start_blk + entry->count))
 			n = &(*n)->rb_right;
 		else {
+			ext4_unlock_group(sb, group);
 			ext4_error(sb, __func__,
 			    "Double free of blocks %d (%d %d)\n",
 			    block, entry->start_blk, entry->count);