More fallout from a77e1f0f81df.

When the tag has an alignment requirement, we should clamp sgsize to be
no larger than the remaining buflen. Otherwise, we will transfer more
than is required, and this can manifest as data corruption.

The issue is observed on powerpc as noted in the pull request:
https://github.com/freebsd/freebsd-src/pull/1415

I also observe the issue locally on riscv hardware, with an 8-byte
transfer having 64-byte alignment.

Try as I might, I cannot articulate the reason for rounding sgsize up to
the tag's alignment, at least in the current code. In fact, it is not
done on arm or arm64 (the latter removed by mmel@ in f635cef2a420).

I speculate that the original purpose of this is now achieved by the line:

sgsize = MIN(buflen, PAGE_SIZE - (curaddr & PAGE_MASK));

I find this code very difficult to reason about, nor is it easy to
experiment with. The fix I have here should be the safest fix, but it
may not be the most correct. This code is fragile and I would benefit
from other knowledgable eyes on it.

Thoughts? What is the correct action to take here?