NFS: Fix a potential deadlock in nfs_release_page

nfs_wb_page() waits on request completion and, as a result, is not safe to be
called from nfs_release_page() invoked by VM scanner as part of GFP_NOFS
allocation. Fix possible deadlock by analyzing gfp mask and refusing to
release page if __GFP_FS is not set.
Signed-off-by: Nikita Danilov <danilov@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
(cherry picked from 374d969debfb290bafcb41d28918dc6f7e43ce31 commit)

fs/nfs/file.c

@@ -312,7 +312,13 @@ static void nfs_invalidate_page(struct page *page, unsigned long offset)
 
 static int nfs_release_page(struct page *page, gfp_t gfp)
 {
+	if (gfp & __GFP_FS)
 		return !nfs_wb_page(page->mapping->host, page);
+	else
+		/*
+		 * Avoid deadlock on nfs_wait_on_request().
+		 */
+		return 0;
 }
 
 const struct address_space_operations nfs_file_aops = {