Make inode64 a remountable option

Actually, there is no reason about why a user must umount and mount a
XFS filesystem to enable 'inode64' option. So, this patch makes this a
remountable option.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>