mount: apply superblock flags from the userns daemon

Superblock flags can be changed only an owner of the global CAP_SYS_ADMIN.

But it is posible to mount tmpfs with any flags.

travis-ci: success for Fix a few issues to dump/restore Docker containers with userns
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>