init: log unmount timeout/failure using KLOG

Log unmount timeout/failure using klog to prevent log data loss during
kernel reboot/shutdown.

It lays the groundwork for log search of these messages in test. The
following CLs: http://ag/c/33041721 and http://ag/c/33061697 search
these log messages for detecting unmount failure.

Bug: 407152708
Test: Revert ag/32872683, reboot & verify unmount failure log in pstore
      console log file(userdebug build)
Change-Id: I5608f3548123bd6de11c6908c751e10dab948ae0