tty: pty: use guard()s

tty: pty: use guard()s

Use guards in the pty code. This improves readability, makes error
handling easier, and marks locked portions of code explicit. All that
while being sure the lock is unlocked.

pty_set_pktmode() is handled specially -- the conditions are inverted
and return called if conditions unmet. This avoid double nested 'if's.
The variable is renamed to want_pktmode so it is not confused with the
current state of pktmode.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://patch.msgid.link/20251119100140.830761-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>