netconsole: move remote_mac from struct netpoll to netconsole_target

The destination ethernet address is netconsole configuration: no other
netpoll user (bonding, team, vlan, bridge, macvlan, dsa) references
np->remote_mac, only netconsole's ethernet framing and its
configfs/cmdline interface do.

Move it into struct netconsole_target and convert push_eth() to take the
netconsole_target; netconsole_print_banner() and
netconsole_parser_cmdline() already take it. The configfs show/store
handlers and alloc_and_init() reach the field directly.

No functional change; the remote_mac sysfs attribute is unchanged.

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260710-netconsole_move_more-v3-9-6f63f76b28bc@debian.org
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>