libxfs: Fix a couple of sparse complaintis

libxfs: Fix a couple of sparse complaintis

No significant changes, just silence a couple of sparse errors.

Using cpu_to_be32(NULLAGINO), the NULLAGINO constant will be encoded in
BE as a constant, avoiding a BE -> CPU conversion every iteraction of
the loop, if be32_to_cpu(agi->agi_unlinked[i]) was used instead.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>