net: fix build with gcc 4.4.7 and strict aliasing

This fix is for IPv6 checksum offload error on RHEL65.
Any optimalisation above -O0 provide error in IPv6 checksum
flag "-fstrict-aliasing" is default for optimalisation above -O0.

Step 1: testpmd -c 0x6 -n 4  -- -i --portmask=0x3 --disable-hw-vlan
                --enable-rx-cksum --crc-strip --txqflags=0
Step 2: settings and start
           set verbose 1
           set fwd csum
           start
Step 3: send scapy with bad checksum IPv6/TCP packet
           Ether(src="52:00:00:00:00:00",
  dst="90:e2:ba:4a:33:5d")/IPv6(src="::1")/TCP(chksum=0xf)/("X"*46)
Step 4: Received packets:
           RESULTS: IPv6/TCP': ['0xd41'] or other unexpected.
  EXPECTED RESULTS: IPv6/TCP': ['0x9f5e']

Fixes: 2b039d5 ("net: fix build with gcc 4.4.7 and strict aliasing")

Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>

lib/librte_net/rte_ip.h

@@ -169,7 +169,8 @@ __rte_raw_cksum(const void *buf, size_t len, uint32_t sum)
 {
 	/* workaround gcc strict-aliasing warning */
 	uintptr_t ptr = (uintptr_t)buf;
-	const uint16_t *u16 = (const uint16_t *)ptr;
+	typedef uint16_t __attribute__((__may_alias__)) u16_p;
+	const u16_p *u16 = (const u16_p *)ptr;
 
 	while (len >= (sizeof(*u16) * 4)) {
 		sum += u16[0];