tty: n_tty: clean up process_output_block()

tty: n_tty: clean up process_output_block()

* Use guard(mutex), which results in:
  - the function can return directly when "space == 0".
  - "i" can now be "unsigned" as it is no longer abused to hold a retval
    from tty->ops->write(). Note the compared-to "nr" is already
    "unsigned".
* The end label is now dubbed "do_write" as that is what happens there.
  Unlike the uncertain "break_out" name.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-8-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>