nfs: Convert to use the preferred fallthrough macro

Convert the uses of fallthrough comments to fallthrough macro. Please see
commit 294f69e6 ("compiler_attributes.h: Add 'fallthrough' pseudo
keyword for switch/case use") for detail.
Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/super.c

@@ -889,7 +889,7 @@ static struct nfs_server *nfs_try_mount_request(struct fs_context *fc)
 		default:
 			if (rpcauth_get_gssinfo(flavor, &info) != 0)
 				continue;
-			/* Fallthrough */
+			fallthrough;
 		}
 		dfprintk(MOUNT, "NFS: attempting to use auth flavor %u\n", flavor);
 		ctx->selected_flavor = flavor;