reboot notifier priority definitions

[Andreas Steinmetz]

As a suggestion to prevent further shutdown/reboot notifier processing problems
here's a (crude) attempt of definitions for include/linux/notifier.h. I do
believe it needs to be heavily reworked by someone with a broad kernel overview.

#define NOTIFY_REBOOT_PHYSICAL    0x00 /* scsi */
#define NOTIFY_REBOOT_LOGICAL     0x10 /* md, lvm */
#define NOTIFY_REBOOT_FS          0x20 /* knfsd */
#define NOTIFY_REBOOT_APPLICATION 0x30 /* tux */

The current situation of not having such definitions already did lead to a
kernel oops (md<->scsi). I do fear that if such definitions are not introduced:

1. code maintainers will strictly stick to priority 0 or will use random
   and not interoperable priorities

2. more and more interference problems will arise due to the above point

Andreas Steinmetz
D.O.M. Datenverarbeitung GmbH