can: isotp: check CAN address family in isotp_bind()

Add missing check to block non-AF_CAN binds.

Syzbot created some code which matched the right sockaddr struct size
but used AF_XDP (0x2C) instead of AF_CAN (0x1D) in the address family
field:

bind$xdp(r2, &(0x7f0000000540)={0x2c, 0x0, r4, 0x0, r2}, 0x10)
                                ^^^^
This has no funtional impact but the userspace should be notified about
the wrong address family field content.

Link: https://syzkaller.appspot.com/text?tag=CrashLog&x=11ff9d8c480000
Reported-by:  <syzbot+5aed6c3aaba661f5b917@syzkaller.appspotmail.com>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20230104201844.13168-1-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>