netconsole: add taskname to extradata entry count

New SYSDATA_TASKNAME feature flag to track when taskname append is enabled.

Additional check in count_extradata_entries() to include taskname in
total, counting it as an entry in extradata. This function is used to
check if we are not overflowing the number of extradata items.

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