kern: add a compressor_method() accessor to the compresssor API

[kevans]

Summary

Some work in progress on the ucoredump bits (D51377) would pass the
compression method out-of-band to the coredump writer's init function,
but the compressor is accessible to the init_fn via the coredump_params.

Making this accessible in parts of the kernel detached from the bits
actually doing compression would be useful to clean up the API so that
we can stick to using coredump_params to pass along whatever may be
useful.

[kevans]

(title is fixed locally, I realized compressor_format() was not a good/consistent name and fixed it just prior to review)