FROMLIST: reboot: Export reboot_mode

Export reboot_mode to support kernel modules wishing to modify reboot_mode.

Change-Id: I8d25aec75331a57fab85c12b8527a329e2a4f19d
Bug: 143964928
Link: https://lore.kernel.org/patchwork/patch/1151092/


Signed-off-by: Elliot Berman <eberman@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>