netlink: Use unsigned type in nl_process_nbuf

[emaste]

Summary

nlmsghdr::nlmsg_len and nl_buf::offset are u_int. Make msglen match.

[glebius]

Original Netlink submission had a ton of violations like that and we are slowly but surely fixing them.

sys/netlink/netlink_io.c

347–352

While here I'd suggest to bring this to style.

[emaste]

var declaration at top of block