kernel/reboot.c: re-enable the function of variable reboot_default

kernel/reboot.c: re-enable the function of variable reboot_default

Commit 1b3a5d02ee07 ("reboot: move arch/x86 reboot= handling to generic
kernel") did some cleanup for reboot= command line, but it made the
reboot_default inoperative.

The default value of variable reboot_default should be 1, and if command
line reboot= is not set, system will use the default reboot mode.

[[email protected]: fix comment layout]
Signed-off-by: Li Fei <[email protected]>
Signed-off-by: liu chuansheng <[email protected]>
Acked-by: Robin Holt <[email protected]>
Cc: <[email protected]>	[3.11.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>