xfs: rework refcount cow recovery error handling

The error handling in xfs_refcount_recover_cow_leftovers is confused
and can potentially leak memory, so rework it to release resources
correctly on error.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reported-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>