[AF_UNIX]: Convert to use a spinlock instead of rwlock

From: Benjamin LaHaise <bcrl@kvack.org>

In af_unix, a rwlock is used to protect internal state.  At least on my 
P4 with HT it is faster to use a spinlock due to the simpler memory 
barrier used to unlock.  This patch raises bw_unix to ~690K/s.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 7d3fe6a..1ddd36d 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -564,7 +564,7 @@
 	u	  = unix_sk(sk);
 	u->dentry = NULL;
 	u->mnt	  = NULL;
-	rwlock_init(&u->lock);
+	spin_lock_init(&u->lock);
 	atomic_set(&u->inflight, sock ? 0 : -1);
 	init_MUTEX(&u->readsem); /* single task reading lock */
 	init_waitqueue_head(&u->peer_wait);