netlink: refactor control data generation for recvmsg(2)

[glebius]

Summary

Netlink should return a very simple control data on every recvmsg(2)
syscall. This data is associated with a syscall, not with an nlmsg,
neither with internal our internal representation (nl_bufs). There is
no need to pre-allocate it in non-sleepable context and attach to
nl_buf. Allocate right in the syscall with M_WAITOK. This also
shaves lots of code and simplifies things.