da(4): Short-circuit unnecessary BIO_FLUSH commands

sys/cam/scsi/scsi_da.c
Complete BIO_FLUSH commands immediately if the da(4) device hasn't
been written to since the last flush. If we haven't written to the
device, there is no reason to send a flush.

Submitted by: gibbs
Reviewed by: imp
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D13106