The NLM_F_ACK flag is ignored for nfnetlink batch begin and end
messages. This is a problem for ynl which wants to receive an ack for
every message it sends, not just the commands in between the begin/end
messages.

Add processing for ACKs for begin/end messages and provide responses
when requested.

I have checked that iproute2, pyroute2 and systemd are unaffected by
this change since none of them use NLM_F_ACK for batch begin/end.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240418104737.77914-5-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>