This is a patch for the megaraid driver that splits the mbox_t structure
into two sections.  The raw_mbox automatics are sized based on the
mbox_out part.  This saves quite a few bytes on the stack (for the
raw_mbox arrays) and limits the amount of bytes in the mbox_t structure
that are cleared.

In issue_scb_block, I changed setting the cmdid and busy elements from
being set in the raw_mbox to being set in the mbox itself after the
memcpy.