xfs: recovery of XLOG_UNMOUNT_TRANS leaks memory

The XLOG_UNMOUNT_TRANS case skips the transaction, despite the fact
an unmount record is always in a standalone transaction. Hence
whenever we come across one of these we need to free the transaction
structure associated with it as there is no commit record that
follows it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>