watchdog: softdog.c: enhancement to optionally invoke panic instead of reboot on timer expiry

This is needed for determining the reason for failure when a softdog
timeout occurs.

We use softdog to watch for critical application failures and at the
minimum a snapshot of the system would help to determine the cause.  In
such a scenario the application could fail but there isn't a softlockup as
such, hence the detect softlockup feature does not help.

The patch adds a module parameter soft_panic which when set to 1 causes
softdog to invoke panic instead of reboot when the softdog timer expires. 
By invoking panic we execute kdump if it is configured and the vmcore
generated by kdump should provide atleast a minimal idea of the reason for
failure.

Based on an original patch by Ken Sugawara <sugaken.r3@gmail.com>
Signed-off-by: Anithra P J <anithra@linux.vnet.ibm.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>