l2arc: reset b_tmp_cdata to NULL in the case of unset b_daddr

The change is in arc_buf_l2_cdata_free().
Without this we can trip the assertion in arc_hdr_realloc()
if INVARIANTS option is enabled.

Approved by:	re (kib)
MFC after:	1 week

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c

@@ -2249,6 +2249,7 @@ arc_buf_l2_cdata_free(arc_buf_hdr_t *hdr)
 		ASSERT3P(hdr->b_l1hdr.b_tmp_cdata, ==,
 		    hdr->b_l1hdr.b_buf->b_data);
 		ASSERT3U(hdr->b_l2hdr.b_compress, ==, ZIO_COMPRESS_OFF);
+		hdr->b_l1hdr.b_tmp_cdata = NULL;
 		return;
 	}