MFC r223084 (by gibbs):

MFC r223084 (by gibbs):
sys/cam/scsi/scsi_da.c:
        - Only attempt the closing synchronize cache on a disk
          if it is still there.
        - When a device is lost, report the number of outstanding
          I/Os as they are drained.
        - When a device is lost, return any unprocessed bios with
          ENXIO instead of EIO.
        - Filter asynchronous events, but always allow cam_periph_async()
          to see them too.

git-svn-id: svn://svn.freebsd.org/base/stable/8@236678 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f