crypto: compress - remove crt_u.compress (struct compress_tfm)

crt_u.compress (struct compress_tfm) is pointless because its two
fields, ->cot_compress() and ->cot_decompress(), always point to
crypto_compress() and crypto_decompress().

Remove this pointless indirection, and just make crypto_comp_compress()
and crypto_comp_decompress() be direct calls to what used to be
crypto_compress() and crypto_decompress().

Also remove the unused function crypto_comp_cast().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>