ipmi: add parameter to limit CPU usage in kipmid

ipmi: add parameter to limit CPU usage in kipmid

In some cases kipmid can use a lot of CPU.  This adds a way to tune the
CPU used by kipmid to help in those cases.  By setting kipmid_max_busy_us
to a value between 100 and 500, it is possible to bring down kipmid CPU
load to practically 0 without loosing too much ipmi throughput
performance.  Not setting the value, or setting the value to zero,
operation is unaffected.

Signed-off-by: Martin Wilck <[email protected]>
Signed-off-by: Corey Minyard <[email protected]>
Cc: Jean Delvare <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>