netlink: provide snl(3) API for variable length raw data attribute

Rename supposedly internal _snl_reserve_msg_attr() into an official
snl_reserve_msg_attr_raw(), that would return pointer to a struct
nlattr followed by allocated memory. Adjust the snl_reserve_msg_attr()
macro to work on top of that function.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D48311