ipmi:msghandler: Export and fix panic messaging capability

Don't have the other users that do things at panic time (the watchdog)
do all this themselves, provide a function to do it.

Also, with the new design where most stuff happens at thread context,
a few things needed to be fixed to avoid doing locking in a panic
context.

Signed-off-by: Corey Minyard <cminyard@mvista.com>