netlink: refactor control data generation for recvmsg(2)

netlink: refactor control data generation for recvmsg(2)

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.

Reviewed by:		melifaro
Differential Revision:	https://reviews.freebsd.org/D42989