nfs: fix compilation warning

this commit fix compilation warning as following:
linux-2.6/fs/nfs/nfs4proc.c:3265: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Jovi Zhang <bookjovi@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index d1ed671..b07d4e2 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -3262,7 +3262,7 @@
 	spages = pages;
 
 	do {
-		len = min(PAGE_CACHE_SIZE, buflen);
+		len = min_t(size_t, PAGE_CACHE_SIZE, buflen);
 		newpage = alloc_page(GFP_KERNEL);
 
 		if (newpage == NULL)