Just serialize the actual writing of packets into pipe on
a new mutex, independent from everything else in the locking
hierarchy.  As soon as something has started feeding a piece
of packet into the pipe to daemon, we *want* everything else
about to try the same to wait until we are done.
Acked-by: NIan Kent <raven@themaw.net>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>