[SCSI] sd: fix computation of the full size of the device

When computing the full size of the device, we need to cast
sdkp->capacity before shifting, since in some configurations sector_t
can be a 32-bit number.

Also, change ffz(~x) to the more idiomatic ilog2(x).

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>