Now that we've separated the startblock and CoW/shared extent domain in
the incore refcount record structure, check the domain whenever we
retrieve a record to ensure that it's still in the domain that we want.
Depending on the circumstances, a change in domain either means we're
done processing or that we've found a corruption and need to fail out.

The refcount check in xchk_xref_is_cow_staging is redundant since
_get_rec has done that for a long time now, so we can get rid of it.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>