NFS: Convert nfs_attr_generation_counter into an atomic_long

The most important property we need from nfs_attr_generation_counter is
monotonicity, which is not guaranteed by the current system of smp memory
barriers. We should convert it to an atomic_long_t, and drop the memory
barriers.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -908,21 +908,16 @@ static int nfs_size_need_update(const struct inode *inode, const struct nfs_fatt
 	return nfs_size_to_loff_t(fattr->size) > i_size_read(inode);
 }
 
-static unsigned long nfs_attr_generation_counter;
+static atomic_long_t nfs_attr_generation_counter;
 
 static unsigned long nfs_read_attr_generation_counter(void)
 {
-	smp_rmb();
-	return nfs_attr_generation_counter;
+	return atomic_long_read(&nfs_attr_generation_counter);
 }
 
 unsigned long nfs_inc_attr_generation_counter(void)
 {
-	unsigned long ret;
-	smp_rmb();
-	ret = ++nfs_attr_generation_counter;
-	smp_wmb();
-	return ret;
+	return atomic_long_inc_return(&nfs_attr_generation_counter);
 }
 
 void nfs_fattr_init(struct nfs_fattr *fattr)