NFS: Fix the resolution problem with nfs_inode_attrs_need_update()

It appears that 'jiffies' timestamps do not have high enough resolution for
nfs_inode_attrs_need_update(). One problem is that a GETATTR can be
launched within < 1 jiffy of the last operation that updated the attribute.
Another problem is that RPC calls can take < 1 jiffy to execute.

We can fix this by switching the variables to use a simple global counter
that gets incremented every time we start another GETATTR call.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_xdr.h

@@ -56,6 +56,7 @@ struct nfs_fattr {
 	__u64			change_attr;	/* NFSv4 change attribute */
 	__u64			pre_change_attr;/* pre-op NFSv4 change attribute */
 	unsigned long		time_start;
+	unsigned long		gencount;
 };
 
 #define NFS_ATTR_WCC		0x0001		/* pre-op WCC data    */