xfrm: avoid spinlock in get_acqseq() used by xfrm user

Eric's version fixed it for pfkey. This one is for xfrm user.
I thought about amortizing those two get_acqseq()s but it seems
reasonable to have two of these sequence spaces for the two different
interfaces.

cheers,
jamal
commit d5168d5addbc999c94aacda8f28a4a173756a72b
Author: Jamal Hadi Salim <hadi@cyberus.ca>
Date:   Tue Feb 16 06:51:22 2010 -0500

    xfrm: avoid spinlock in get_acqseq() used by xfrm user

    This is in the same spirit as commit 28aecb9d77
    by Eric Dumazet.
    Use atomic_inc_return() in get_acqseq() to avoid taking a spinlock

    Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

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

net/xfrm/xfrm_state.c

@@ -1462,12 +1462,12 @@ EXPORT_SYMBOL(xfrm_find_acq_byseq);
 u32 xfrm_get_acqseq(void)
 {
 	u32 res;
-	static u32 acqseq;
-	static DEFINE_SPINLOCK(acqseq_lock);
+	static atomic_t acqseq;
+
+	do {
+		res = atomic_inc_return(&acqseq);
+	} while (!res);
 
-	spin_lock_bh(&acqseq_lock);
-	res = (++acqseq ? : ++acqseq);
-	spin_unlock_bh(&acqseq_lock);
 	return res;
 }
 EXPORT_SYMBOL(xfrm_get_acqseq);