lib: Extract from devlink/mnlg a helper, mnlu_msg_prepare()

lib: Extract from devlink/mnlg a helper, mnlu_msg_prepare()

Allocation of a new netlink message with the two usual headers is reusable
with other netlink netlink message types. Extract it into a helper,
mnlu_msg_prepare(). Take the second header as an argument, instead of
passing in parameters to initialize it, and copy it in.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>