fdstore: add synchronization to protect a peek offset

When we need to get a file descriptor, we have to set peek offset
and then to peek a message. There are two system calls, so we need
to be sure that nobody changes a socket peek offset after we set it.

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