mount: Use issubpath() when checking for submount visibility

When we check whether a submount of a mount is visible in another
mount (shared peer of the latter), we can and should use the new
issubpath helper.

Should because the used strncmp may scan beyond ct_mpnt_rpath if
its length is smaller (no checks for this in the code).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>