xfs: refactor unmount record detection into helper

Once the mount sequence has identified the head and tail blocks of the
physical log, the record at the head of the log is located and examined
for an unmount record to determine if the log is clean. This currently
occurs after torn write verification of the head region of the log.

This must ultimately be separated from torn write verification and may
need to be called again if the log head is walked back due to a torn
write (to determine whether the new head record is an unmount record).
Separate this logic into a new helper function. This patch does not
change behavior.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>