xfs: Do not return EFSCORRUPTED when filesystem probe finds no XFS magic

98021821 changed the return value from EWRONGFS (aka EINVAL)
to EFSCORRUPTED which doesn't seem to be handled properly by
the root filesystem probe.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Tested-by: Sergei Trofimovich <slyfox@gentoo.org>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>