[PATCH] ipmi: si start transaction hook

Some commands, on some system BMCs, don't respond at at all.  This is seen on
Dell PowerEdge x6xx and x7xx systems with IPMI 1.0 BT controllers when a "Get
SDR" command is issued, with a length field of 0x3A, which happens to be the
length of about SDR entries.  If another length is passed, this command
succeeds.

This patch adds general infrastructure for receiving commands before they're
passed down to the low-level drivers, such that they can be completed
immediately, or modified, prior to being sent to ->start_transaction().

Signed-off-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>