ggml-cpu: repack: Fix chunks being too small with small matrix shapes in REPACK forward_mul_mat

[Alcpz]

For small shapes where the number of columns is small (i.e. 16), the current logic skipped some chunks due to rounding.

The issue was observed with NB_COLS 8 and ne01 16, and could potentially happen with NB_COLS 4 and other combinations threads/shape.
This is also affected the corner case where chunking is disabled.

@max-krasnyansky I checked the performance here and didn't see any issue. Let me know if you'd like me to perform any particular test

Performance

RPI5

model	test	2f416b2 (7162) t/s	3e18dba (7161) t/s
lfm2 350M Q4_0	pp256	174.46 ± 0.07	173.41 ± 0.64
lfm2 350M Q4_0	tg128	51.58 ± 0.03	51.38 ± 0.26
lfm2 700M Q4_0	pp256	81.79 ± 0.01	82.55 ± 0.03
lfm2 700M Q4_0	tg128	25.78 ± 0.00	25.86 ± 0.00

M4 max

model	test	2f416b2 (7162) t/s	3e18dba (7161) t/s
lfm2 1.2B Q4_K Medium	pp256	682.39 ± 3.23	682.82 ± 2.97
lfm2 1.2B Q4_K Medium	tg128	233.77 ± 4.45	234.96 ± 0.57
lfm2 700M Q4_K Medium	pp256	1070.08 ± 2.77	1067.29 ± 7.14
lfm2 700M Q4_K Medium	tg128	331.12 ± 1.27	333.13 ± 1.32
llama 8B Q4_K Medium	pp256	100.26 ± 0.11	96.65 ± 1.75
llama 8B Q4_K Medium	tg128	43.10 ± 0.50	41.69 ± 0.72
qwen3 8B Q4_K Medium	pp256	94.40 ± 0.33	90.45 ± 0.34
qwen3 8B Q4_K Medium	tg128	40.92 ± 0.33	40.29 ± 0.27

[max-krasnyansky]

Looks good to me. It's funny how many little corner cases we ended up having to deal with.
The original logic I added (ie 4x chunks per thread) seemed so simple and bulletproof :)

Tested on my Snapdragon Gen5 with a bunch of models (llama-3.2-1/2B, qwen3-0.6B .. 8B, LFM2s, ...).
nchunk selection looks good and the overall performance is the same. Merging ...

[Alcpz]

Yeah totally. I guess these smaller cases are not representative of the models that are out there and that's why we don't run into them. Thanks for the review and merge