nfs_common: fix build warning in grace.c

This fix the following warning
fs/nfs_common/grace.c:66:1: warning: no previous prototype for function '__state_in_grace' [-Wmissing-prototypes]
by adding the missing static.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfs_common/grace.c

@@ -62,7 +62,7 @@ EXPORT_SYMBOL_GPL(locks_end_grace);
  * to answer ordinary lock requests, and when they should accept only
  * lock reclaims.
  */
-int
+static int
 __state_in_grace(struct net *net, bool open)
 {
 	struct list_head *grace_list = net_generic(net, grace_net_id);