tty: tty_buffer: warn if losing flags in __tty_insert_flip_string_flags()

And add a WARN_ON_ONCE(need_flags) to make sure we are not losing flags
in __tty_insert_flip_string_flags().

Signed-off-by: "Jiri Slaby (SUSE)" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_buffer.c

···
 			flags += space;
 		} else if (tb->flags) {
 			memset(flag_buf_ptr(tb, tb->used), flags[0], space);
+		} else {
+			/* tb->flags should be available once requested */
+			WARN_ON_ONCE(need_flags);
 		}
 
 		tb->used += space;