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>
MFC after:	1 week

(cherry picked from commit fe221d29bf0f99dc465137bf6fea3a92d8306375)