tty: tty_jobctrl: use guard() in tiocgsid()

guard()s express more clearly what the lock protects and let the
function return immediately instead of jumping to an unlock label.

Signed-off-by: John de la Garza <john@jjdev.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://patch.msgid.link/20260701231215.4092457-1-john@jjdev.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>