Bluetooth: RFCOMM: Fix not validating setsockopt user input

syzbot reported rfcomm_sock_setsockopt_old() is copying data without
checking user input length.

BUG: KASAN: slab-out-of-bounds in copy_from_sockptr_offset
include/linux/sockptr.h:49 [inline]
BUG: KASAN: slab-out-of-bounds in copy_from_sockptr
include/linux/sockptr.h:55 [inline]
BUG: KASAN: slab-out-of-bounds in rfcomm_sock_setsockopt_old
net/bluetooth/rfcomm/sock.c:632 [inline]
BUG: KASAN: slab-out-of-bounds in rfcomm_sock_setsockopt+0x893/0xa70
net/bluetooth/rfcomm/sock.c:673
Read of size 4 at addr ffff8880209a8bc3 by task syz-executor632/5064

Fixes: 9f2c8a03 ("Bluetooth: Replace RFCOMM link mode with security level")
Fixes: bb23c0ab

 ("Bluetooth: Add support for deferring RFCOMM connection setup")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>