ipmi:watchdog: Rework locking and handling

Simplify things by creating one set of message handling data for
setting the watchdog and doing a heartbeat.  Rework the locking
to avoid some (probably not very important) races and to avoid
a fairly unlikely infinite recursion.

Get rid of ipmi_ignore_heartbeat, it wasn't used, and use
watchdog_user to tell if we have a working IPMI device below
us.
Signed-off-by: Corey Minyard <cminyard@mvista.com>