Add wrapper for Linux BLKFLSBUF ioctl

FreeBSD has no analog. Buffered block devices were removed a decade
plus ago.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9508

cmd/zdb/zdb.c

@@ -3444,7 +3444,7 @@ dump_label(const char *dev)
 		exit(1);
 	}
 
-	if (S_ISBLK(statbuf.st_mode) && ioctl(fd, BLKFLSBUF) != 0)
+	if (S_ISBLK(statbuf.st_mode) && zfs_dev_flush(fd) != 0)
 		(void) printf("failed to invalidate cache '%s' : %s\n", path,
 		    strerror(errno));
 

cmd/zpool/zpool_main.c

@@ -1178,7 +1178,7 @@ zpool_do_labelclear(int argc, char **argv)
 	 * fatal when the device does not support BLKFLSBUF as would be the
 	 * case for a file vdev.
 	 */
-	if ((ioctl(fd, BLKFLSBUF) != 0) && (errno != ENOTTY))
+	if ((zfs_dev_flush(fd) != 0) && (errno != ENOTTY))
 		(void) fprintf(stderr, gettext("failed to invalidate "
 		    "cache for %s: %s\n"), vdev, strerror(errno));