kernel/reboot: explicitly notify if halt occurred instead of power off

When kernel_can_power_off() returns false, and reboot has called with
LINUX_REBOOT_CMD_POWER_OFF, kernel_halt() will be initiated instead of
actual power off function.

However, in this situation, Kernel never explicitly notifies user that
system halted instead of requested power off.

Since halt and power off perform different behavior, and user initiated
reboot call with power off command, not halt, This could be unintended
behavior to user, like this:

~ # poweroff -f
[    3.581482] reboot: System halted

Therefore, this explicitly notifies user that poweroff is not available,
and halting has been occured as an alternative behavior instead:

~ # poweroff -f
[    4.123668] reboot: Power off not available: System halted instead

[akpm@linux-foundation.org: tweak comment text]
Link: https://lkml.kernel.org/r/20231104113320.72440-1-ldmldm05@gmail.com
Signed-off-by: Dongmin Lee <ldmldm05@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>