fs/affs: make root lookup from blkdev logical size

This patch resolves https://bugzilla.kernel.org/show_bug.cgi?id=16531

.

When logical blkdev size > 512 then sector numbers become larger than the
device can support.

Make affs start lookup based on the device's logical sector size instead
of 512.

Reported-by: Mark <markk@clara.co.uk>
Suggested-by: Mark <markk@clara.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>