net: move compute_mcast_idx() to net.h

Reduce duplicated codes.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/net.h b/net.h
index 75a8c15..64993b4 100644
--- a/net.h
+++ b/net.h
@@ -182,6 +182,9 @@
 
 int net_handle_fd_param(Monitor *mon, const char *param);
 
+#define POLYNOMIAL 0x04c11db6
+unsigned compute_mcast_idx(const uint8_t *ep);
+
 #define vmstate_offset_macaddr(_state, _field)                       \
     vmstate_offset_array(_state, _field.a, uint8_t,                \
                          sizeof(typeof_field(_state, _field)))