xfs_repair: use libxfs extsize/cowextsize validation helpers

xfs_repair: use libxfs extsize/cowextsize validation helpers

Now that we've ported the extent size hint verifiers to libxfs, call
them from xfs_repair instead of open-coding the checks.  Tweak the
repair message slightly to reflect the fact that we zero the field and
clear the inode flag if the hint is garbage or is inconsistent with the
inode flags.

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