libzfs: Zero DIOCLWEDGES input before ioctl.

Otherwise we ask the kernel to write over whatever random pointer was
in the stack garbage here, when all we wanted was to learn whether
dkwl_nwedges is zero or nonzero.

XXX pullup-9

external/cddl/osnet/dist/lib/libzfs/common/libzfs_import.c

@@ -1099,6 +1099,7 @@ zpool_open_func(void *arg)
 	off_t size;
 
 	/* skip devices with wedges */
+	memset(&dkwl, 0, sizeof(dkwl));
 	if (native_ioctl(fd, DIOCLWEDGES, &dkwl) == 0 &&
 	    dkwl.dkwl_nwedges > 0) {
 		(void) close(fd);