tcp_metrics: fix tcp_metrics_nl_dump() return value

Change tcp_metrics_nl_dump() to return 0 at the end
of a dump so that NLMSG_DONE can be appended
to the current skb, saving one recvmsg() system call.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240416161112.1199265-1-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>