net: avoid cast-align warning in VLAN insert function

net: avoid cast-align warning in VLAN insert function

In rte_vlan_insert there is a casting of rte_pktmbuf_prepend returned
value to (struct rte_ether_hdr *), which causes cast-align warning when
using strict cast align flag with supporting gcc:
gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
CFLAGS="-Wcast-align=strict" make V=1 -C examples/l2fwd clean static

In file included from main.c:35:
/dpdk/build/include/rte_ether.h:370:7: warning: cast increases required
alignment of target type [-Wcast-align]
  370 |  nh = (struct rte_ether_hdr *)
      |       ^

As the code assumes correct alignment, add first a (void *) casting, to
avoid the warning.

Fixes: c974021a5949 ("ether: add soft vlan encap/decap")
Cc: stable@dpdk.org
Signed-off-by: Eli Britstein <elibr@nvidia.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>