NFS: Use an atomic_long_t to count the number of requests

Rather than forcing us to take the inode->i_lock just in order to bump
the number.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/callback_proc.c b/fs/nfs/callback_proc.c
index 5427cdf..14358de 100644
--- a/fs/nfs/callback_proc.c
+++ b/fs/nfs/callback_proc.c
@@ -51,7 +51,7 @@
 		goto out_iput;
 	res->size = i_size_read(inode);
 	res->change_attr = delegation->change_attr;
-	if (nfsi->nrequests != 0)
+	if (nfs_have_writebacks(inode))
 		res->change_attr++;
 	res->ctime = inode->i_ctime;
 	res->mtime = inode->i_mtime;