smb: client: fix compression heuristic functions

Change is_compressible() return type to bool, use WARN_ON_ONCE(1) for
internal errors and return false for those.

Renames:
check_repeated_data -> has_repeated_data
check_ascii_bytes -> is_mostly_ascii (also refactor into a single loop)
calc_shannon_entropy -> has_low_entropy

Also wraps "wreq->Length" in le32_to_cpu() in should_compress() (caught
by sparse).

Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Steve French <stfrench@microsoft.com>