reboot: remove cf9_safe from allowed types and rename cf9_force

BOOT_CF9_SAFE_STR is an internal value used only by the x86 code and it's
not possible to set it from userspace.

Remove it, and rename 'cf9_force' to 'pci', so to make it coherent with
the kernel command line reboot= option.

Tested with this script:

	cd /sys/kernel/reboot/

	for i in cold warm hard soft gpio; do
		echo $i >mode
		read j <mode
		[ $i = $j ] || echo "mode $i != $j"
	done

	for i in bios acpi kbd triple efi pci; do
		echo $i >type
		read j <type
		[ $i = $j ] || echo "type $i != $j"
	done

	for i in $(seq 0 $(nproc --ignore=1)); do
		echo $i >cpu
		read j <cpu
		[ $i = $j ] || echo "cpu $i != $j"
	done

	for i in 0 1; do
		echo $i >force
		read j <force
		[ $i = $j ] || echo "force $i != $j"
	done

Link: https://lkml.kernel.org/r/20201113015900.543923-1-mcroce@linux.microsoft.com


Fixes: eab8da48579d ("reboot: allow to specify reboot mode via sysfs")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>