netconsole: add task name to extra data fields

This is the core patch for this whole patchset. Add support for
including the current task's name in netconsole's extra data output.
This adds a new append_taskname() function that writes the task name
(from current->comm) into the target's extradata buffer, similar to how
CPU numbers are handled.

The task name is included when the SYSDATA_TASKNAME field is set,
appearing in the format "taskname=<name>" in the output. This additional
context can help with debugging by showing which task generated each
console message.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>