xfs_scrub: handle scrub-only kernels more helpfully

xfs_scrub: handle scrub-only kernels more helpfully

If xfs_scrub is run today against a 4.15 kernel, it fails with

EXPERIMENTAL xfs_scrub program in use! Use at your own risk!
Error: /home: Kernel metadata optimization facility is required.
Info: /home: Scrub aborted after phase 1.
/home: 2 errors found.

Be a bit kinder to the user and suggest a path forward.  By the
time we fail for missing preen or repair functionality, we do
know that scrub is available, so suggest it.

Further, rather than stating what is required, state what was not
found ... we're failing, so state what was missing, vs. what is
required - seems a bit more definitive.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>