9p: fix spelling mistake in fall-through annotation

Replace "fallthough" with a proper "fall through" annotation.

This fix is part of the ongoing efforts to enabling -Wimplicit-fallthrough

Link: http://lkml.kernel.org/r/20180903193806.GA11258@embeddedor.com
Addresses-Coverity-ID: 402012 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>

fs/9p/vfs_file.c

@@ -216,7 +216,7 @@ static int v9fs_file_do_lock(struct file *filp, int cmd, struct file_lock *fl)
 		break;
 	default:
 		WARN_ONCE(1, "unknown lock status code: %d\n", status);
-		/* fallthough */
+		/* fall through */
 	case P9_LOCK_ERROR:
 	case P9_LOCK_GRACE:
 		res = -ENOLCK;