reboot: allow to override reboot type if quirks are found

Patch series "reboot: sysfs improvements".

Some improvements to the sysfs reboot interface: hide not working settings
and support machines with known reboot quirks.

This patch (of 2):

On some machines a quirk can force a specific reboot type.  Quirks are
found during a DMI scan, the list of machines which need special reboot
handling is defined in reboot_dmi_table.

The kernel command line reboot= option overrides this via a global
variable `reboot_default`, so that the reboot type requested in the
command line is really performed.

This was not true when setting the reboot type via the new sysfs
interface.  Fix this by setting reboot_default upon the first change, like
reboot_setup() does for the command line.

Link: https://lkml.kernel.org/r/20201130173717.198952-1-mcroce@linux.microsoft.com
Link: https://lkml.kernel.org/r/20201130173717.198952-2-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>