The FS_TRIM fs flag indicates that administrator requested issuing of
TRIM commands for the volume. UFS will only send the command to disk
if the disk reports GEOM::candelete attribute.
Since disk queue is reordered, data block is marked as free in the bitmap
only after TRIM command completed. Due to need to sleep waiting for
i/o to finish, TRIM bio_done routine schedules taskqueue to set the
bitmap bit.
Based on the patch by: mckusick
Reviewed by: mckusick, pjd
Tested by: pho
MFC after: 1 month
8c2a54de80