mount: don't add a separate entry for each enabled file systems

It isn't required and we will not have a limit for enabled file systems.

v2: clean up
v3: add a shortcut to get a mount file system
v4: fix comments from Pavel

Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>