[RFC] crypto: passing configuration parameters to comp algos

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Hi,

	We'd like to be able to pass algorithm-specific parameters to
comp backends. As of this moment, crypto usees hard-coded default
values and does not permit any run-time algorithm configuration,
which in some cases simply disables the most interesting functionality.
E.g. zstd can be configured to use a pre-trained (in the user-space)
compression dictionary, which significantly changes algorithms
characteristics. Another, obvious and trivial example, is algorithms
compression level.

The problem is that we need to pass params to cra_init() function,
because for some algorithms that's the only place where configuration
can take place (e.g. zstd). Changing cra_init() to accept additional
`struct crypto_comp_params` looks to be a little intrusive so before
I write any patches I'd like to hear your thoughts.