net: udp: remove redundant initialization in udp_dump_one

The initialization for 'err' with '-EINVAL' is redundant and
can be removed, as it is updated soon and not used.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Link: https://lore.kernel.org/r/1604644960-48378-2-git-send-email-dong.menglong@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>