netconsole: refactor CPU number formatting into separate function

Extract CPU number formatting logic from prepare_extradata() into a new
append_cpu_nr() function.

This refactoring improves code organization by isolating CPU number
formatting into its own function while reducing the complexity of
prepare_extradata().

The change prepares the codebase for the upcoming taskname feature by
establishing a consistent pattern for handling sysdata features.

The CPU number formatting logic itself remains unchanged; only its
location has moved to improve maintainability.

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