btrfs: fix missing snapshot drew unlock when root is dead during swap activation

When activating a swap file we acquire the root's snapshot drew lock and
then check if the root is dead, failing and returning with -EPERM if it's
dead but without unlocking the root's snapshot lock. Fix this by adding
the missing unlock.

Fixes: 60021bd ("btrfs: prevent subvol with swapfile from being deleted")
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Filipe Manana <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/inode.c

@@ -9876,6 +9876,7 @@ static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
 	if (btrfs_root_dead(root)) {
 		spin_unlock(&root->root_item_lock);
 
+		btrfs_drew_write_unlock(&root->snapshot_lock);
 		btrfs_exclop_finish(fs_info);
 		btrfs_warn(fs_info,
 		"cannot activate swapfile because subvolume %llu is being deleted",