ipmi: Add a sysfs count of total outstanding messages for an interface

Go through each user and add its message count to a total and print the
total.

It would be nice to have a per-user file, but there's no user sysfs
entity at this point to hang it off of.  Probably not worth the effort.

Based on work by Chen Guanqiao <chen.chenchacha@foxmail.com>

Cc: Chen Guanqiao <chen.chenchacha@foxmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>