netlink: change size types from unsigned long to kernel_ureg_t

netlink: change size types from unsigned long to kernel_ureg_t

* defs.h (decode_netlink): Change len argument type from unsigned long
to kernel_ureg_t.
* netlink.c (fetch_nlmsghdr): Likewise.
(decode_nlmsghdr_with_payload): Likewise.  Change nlmsg_len variable
type from unsigned long to unsigned int.
(decode_netlink): Change the type of len argument and next_len variable
from unsigned long to kernel_ureg_t.  Change nlmsg_len variable type
from unsigned long to unsigned int.