net: Fix soft lockups/OOM issues w/ unix garbage collector

This is an implementation of David Miller's suggested fix in:
  https://bugzilla.redhat.com/show_bug.cgi?id=470201

It has been updated to use wait_event() instead of
wait_event_interruptible().

Paraphrasing the description from the above report, it makes sendmsg()
block while UNIX garbage collection is in progress. This avoids a
situation where child processes continue to queue new FDs over a
AF_UNIX socket to a parent which is in the exit path and running
garbage collection on these FDs. This contention can result in soft
lockups and oom-killing of unrelated processes.

Signed-off-by: dann frazier <dannf@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>