xfs: issue log message on user force shutdown

The kernel only issues a log message that it's been shut down when
the filesystem triggers a shutdown itself. Hence there is no trace
in the log when a shutdown is triggered manually from userspace.
This can make it hard to see sequence of events in the log when
things go wrong, so make sure we always log a message when a
shutdown is run.

While there, clean up the logic flow so we don't have to continually
check if the shutdown trigger was user initiated before logging
shutdown messages.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>