vfs_read_dirent: increment ncookies after adding a cookie

It seems that at present vfs_read_dirent() is used only with filesystems
that do not support cookies, so the bug never manifested itself.

MFC after:	1 week

sys/kern/vfs_subr.c

@@ -5053,6 +5053,7 @@ vfs_read_dirent(struct vop_readdir_args *ap, struct dirent *dp, off_t off)
 	*ap->a_cookies = realloc(*ap->a_cookies,
 	    (*ap->a_ncookies + 1) * sizeof(u_long), M_TEMP, M_WAITOK | M_ZERO);
 	(*ap->a_cookies)[*ap->a_ncookies] = off;
+	*ap->a_ncookies += 1;
 	return (0);
 }