NFSv4.2: fix error handling in nfs42_proc_getxattr

There is a slight issue with error handling code inside
nfs42_proc_getxattr(). If page allocating loop fails then we free the
failing page array element which is NULL but __free_page() can't deal with
NULL args.

Found by Linux Verification Center (linuxtesting.org).

Fixes: a1f26739 ("NFSv4.2: improve page handling for GETXATTR")
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs42proc.c

@@ -1377,7 +1377,6 @@ ssize_t nfs42_proc_getxattr(struct inode *inode, const char *name,
 	for (i = 0; i < np; i++) {
 		pages[i] = alloc_page(GFP_KERNEL);
 		if (!pages[i]) {
-			np = i + 1;
 			err = -ENOMEM;
 			goto out;
 		}
@@ -1401,8 +1400,8 @@ ssize_t nfs42_proc_getxattr(struct inode *inode, const char *name,
 	} while (exception.retry);
 
 out:
-	while (--np >= 0)
-		__free_page(pages[np]);
+	while (--i >= 0)
+		__free_page(pages[i]);
 	kfree(pages);
 
 	return err;