mount: Allow to c/r empty mqueues

We don't support posix mqueues at the moment
but in case if this fs is simply mounted and
not used lets proceed without errors.

In case if someone is using it we detect it
because fs won't be empty and refuse to dump.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>