[SCSI] ipr: Increase alignment boundary of command blocks

The latest generation of ipr hardware performs best when command blocks
are aligned to a boundary equal to the size of the command block. Ensure
512 byte alignment, since this is the largest size command block we
can send.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>