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

lib/libzutil/os/linux/zutil_import_os.c

@@ -92,6 +92,12 @@ is_watchdog_dev(char *dev)
 	return (B_FALSE);
 }
 
+int
+zfs_dev_flush(int fd)
+{
+	return (ioctl(fd, BLKFLSBUF));
+}
+
 void
 zpool_open_func(void *arg)
 {