tty: tty_buffer: let tty_prepare_flip_string() return size_t

The same as in the previous patch, tty_prepare_flip_string() accepts
size_t as an size argument. It returns the same size (or less). It is
unexpected that it returns a signed value and can confuse users to check
for negative values.

Instead, return the same size_t as accepted to make clear we return
values >= 0, where zero in fact means failure.

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

···
  * Returns: the length available and buffer pointer (@chars) to the space which
  * is now allocated and accounted for as ready for normal characters.
  */
-int tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size)
+size_t tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size)
 {
-	int space = __tty_buffer_request_room(port, size, false);
+	size_t space = __tty_buffer_request_room(port, size, false);
 
 	if (likely(space)) {
 		struct tty_buffer *tb = port->buf.tail;
···
 			memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
 		tb->used += space;
 	}
+
 	return space;
 }
 EXPORT_SYMBOL_GPL(tty_prepare_flip_string);

include/linux/tty_flip.h

···
 size_t __tty_insert_flip_string_flags(struct tty_port *port, const u8 *chars,
 				      const u8 *flags, bool mutable_flags,
 				      size_t size);
-int tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size);
+size_t tty_prepare_flip_string(struct tty_port *port, u8 **chars, size_t size);
 void tty_flip_buffer_push(struct tty_port *port);
 int __tty_insert_flip_char(struct tty_port *port, u8 ch, u8 flag);