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: NJovi Zhang <bookjovi@gmail.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -3262,7 +3262,7 @@ static int buf_to_pages_noslab(const void *buf, size_t buflen,
 	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)