cache: avoid hardcoded CACHE_PATH_CUTOFF

[brooks]

Summary

Compute the cutoff at compile time instead which will avoid the need
for a CHERI case.

No functional change.

Suggested by: jhb
Effort: CHERI upstreaming
Sponsored by: DARPA, AFRL

[markj]

Looks ok to me otherwise.

sys/kern/vfs_cache.c

409–418

This comment is a bit confusing now. You might list the actual values for CACHE_PATH_CUTOFF here.

[brooks]

Update the CACHE_PATH_CUTOFF comment adding more context and values.

[markj]

sys/kern/vfs_cache.c

410

[olce]

sys/kern/vfs_cache.c

409–418

An initial value of 39 cannot be bumped later into 39, something is inconsistent here.

Some digging shows that there was no such cutoff as the existing one in the 4.4 BSD import in the sense that only names shorter than 31 chars were cached, the others were not. 39 appeared after several bumps, and after introduction of the small/large split (and caching of all names).

I'd suggest eliding all but recent history, since that part seems wrong.

[brooks]

Fix up comment removing errors