cache: avoid hardcoded cache padding

[brooks]

Summary

Replace CACHE_LARGE_PAD with rounding up to the alignment of struct
namecache_ts.

No functional change.

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

[olce]

I fail to see any usefulness of CACHE_LARGE_PAD when it was introduced, which corresponds to aligning CACHE_ZONE_LARGE_SIZE to struct namecache_ts. So I'd just drop the corresponding roundup2(), with the benefit of not requiring a struct namecache_ts alignment for CACHE_ZONE_LARGE_SIZE (but then you have to change the corresponding static assertion).

[brooks]

In D54553#1247465, @olce wrote:

I fail to see any usefulness of CACHE_LARGE_PAD when it was introduced, which corresponds to aligning CACHE_ZONE_LARGE_SIZE to struct namecache_ts. So I'd just drop the corresponding roundup2(), with the benefit of not requiring a struct namecache_ts alignment for CACHE_ZONE_LARGE_SIZE (but then you have to change the corresponding static assertion).

I'm also skeptical (and think both SMALL_SIZE and LARGE_SIZE are over aligned on e.g., armv7), but I'm wary of making functional changes here.

[olce]

In D54553#1247786, @brooks wrote:

I'm also skeptical (and think both SMALL_SIZE and LARGE_SIZE are over aligned on e.g., armv7), but I'm wary of making functional changes here.

Agreed. If we attempt some such change, it would be more prudent to do it in another revision. I might try to do it, if you don't plan more changes in the area.