MFC r262886:
Fix support for increased logical sector size (4K-native drives).

- Logical sector size is measured in words, not bytes.
- If physical sector is not bigger then logical sector, it does not mean
it should be set equal to 512 bytes, but set to logical sector.

PR:             misc/187269
Submitted by:   Ravi Pokala <rpokala@panasas.com>