Btrfs: fix a double free on pending snapshots in error handling

When creating a snapshot, failing to commit a transaction can end up
with aborting the transaction, following by doing a cleanup for it, where
we'll free all snapshots pending to disk.

So we check it and avoid double free on pending snapshots.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>