Fix rounding writes up to sector size

Fix rounding writes up to sector size

Do this at two levels, although one would be enough to
fix the problem seen recently:

- Ignore any reported sector size other than 512 of 4096.
  If either sector size (physical or logical) is reported
  as 512, then use 512.  If neither are reported as 512,
  and one or the other is reported as 4096, then use 4096.
  If neither is reported as either 512 or 4096, then use 512.

- When rounding up a limited write in bcache to be a multiple
  of the sector size, check that the resulting write size is
  not larger than the bcache block itself.  (This shouldn't
  happen if the sector size is 512 or 4096.)