check: mnt_id is now in mainline

As of
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=49d063cb3


the mnt_id field is in mainline, so let's check for it in --ms.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>