smb: client: compress: rework is_compressible()

Biggest caller-facing change:
is_compressible() is now called from smb_compress() instead of
should_compress();  the latter only does the basic checks (write >= 4K).

Internal changes:
- has_low_entropy() computes Shannon entropy of the sampled data.
Original code was copied over from btrfs, so it carelessly used their
parameters.

Fix the calculation to use our own parameters.
Also set a more relaxed threshold since LZ77 can now handle better
hard-to-compress data.

- The above change also allows to remove calc_byte_distribution()
that did similar work (with more inaccurate results).

- Also rework on sample collection from the input (uncompressed)
buffer; instead of 'sampling', with intervals, now just reuse the
to-be-compressed buffer and run is_compressible() on 7/8 of its
original size.  This runs just as fast and gives much more accurate
info.

- Remove collect_sample() and collect_step() helpers

Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>

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