Honor zfs_nocacheflush for file vdevs

For consistency with disk vdevs honor the zfs_nocacheflush tunable.
This setting is available primarily for debugging and performance
analysis.

Signed-off-by: HC <mmttdebbcc@yahoo.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2336

module/zfs/vdev_file.c

@@ -176,6 +176,10 @@ vdev_file_io_start(zio_t *zio)
 
 		switch (zio->io_cmd) {
 		case DKIOCFLUSHWRITECACHE:
+
+			if (zfs_nocacheflush)
+				break;
+
 			zio->io_error = VOP_FSYNC(vf->vf_vnode, FSYNC | FDSYNC,
 			    kcred, NULL);
 			break;