net/udp: Fix hybrid dual-stack IPv6/IPv4 socket

- Fix `ip6_map_ipv4addr` and `ip6_get_ipv4addr` macro to work under
  different endianness.
- Use `iob_reserve` instead of `iob_trimhead` in `udp_datahandler`.
  - Because we may set `sockaddr_in6` into IPv4 header, which causes
    `offset` become negative. `iob_reserve` can hold this case while
    `iob_trimhead` cannot.
- Select IPv4 domain in send case.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

include/nuttx/net/ip.h

@@ -374,9 +374,8 @@ extern "C"
   do \
     { \
       memset(ipv6addr, 0, 5 * sizeof(uint16_t)); \
-      ipv6addr[5] = 0xffff; \
-      ipv6addr[6] = (uint16_t)((uint32_t)ipv4addr >> 16); \
-      ipv6addr[7] = (uint16_t)ipv4addr & 0xffff; \
+      (ipv6addr)[5] = 0xffff; \
+      net_ipv4addr_hdrcopy(&(ipv6addr)[6], &(ipv4addr)); \
     } \
   while (0)
 
@@ -395,11 +394,7 @@ extern "C"
  *
  ****************************************************************************/
 
-#define ip6_get_ipv4addr(ipv6addr) \
-  (((in_addr_t)(ipv6addr)->s6_addr[12]) | \
-   ((in_addr_t)(ipv6addr)->s6_addr[13] << 8) | \
-   ((in_addr_t)(ipv6addr)->s6_addr[14] << 16) | \
-   ((in_addr_t)(ipv6addr)->s6_addr[15] << 24))
+#define ip6_get_ipv4addr(ipv6addr) net_ip4addr_conv32(&(ipv6addr)[6])
 
 /****************************************************************************
  * Macro: ip6_is_ipv4addr