mbuf: fix strict aliasing in allocator

When building an application with -fstrict-aliasing -Wstrict-aliasing=2,
we get errors triggered by rte_mbuf_raw_alloc() which is called inline
from rte_pktmbuf_alloc().

 ../dpdk/lib/mbuf/rte_mbuf.h: In function ‘rte_mbuf_raw_alloc’:
 ../dpdk/lib/mbuf/rte_mbuf.h:600:42: error: dereferencing type-punned
 pointer might break strict-aliasing rules [-Werror=strict-aliasing]
   600 |         if (rte_mempool_get(mp, (void **)&m) < 0)
       |                                          ^~

Avoid incorrect casting by using an inline union variable.

Signed-off-by: Robin Jarry <rjarry@redhat.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Tested-by: Ali Alnubani <alialnu@nvidia.com>

lib/mbuf/rte_mbuf.h

@@ -595,12 +595,15 @@ __rte_mbuf_raw_sanity_check(__rte_unused const struct rte_mbuf *m)
  */
 static inline struct rte_mbuf *rte_mbuf_raw_alloc(struct rte_mempool *mp)
 {
-	struct rte_mbuf *m;
+	union {
+		void *ptr;
+		struct rte_mbuf *m;
+	} ret;
 
-	if (rte_mempool_get(mp, (void **)&m) < 0)
+	if (rte_mempool_get(mp, &ret.ptr) < 0)
 		return NULL;
-	__rte_mbuf_raw_sanity_check(m);
-	return m;
+	__rte_mbuf_raw_sanity_check(ret.m);
+	return ret.m;
 }
 
 /**