mnt: do_new_mount should restore sharing

The function masks off sharing in the initial setting, but doesn't correctly
reset the flags afterwards.

CC: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Looks-good-to: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>