bpf: avoid VLAs in progs/test_xdp_dynptr.c

VLAs are not supported by either the BPF port of clang nor GCC.  The
selftest test_xdp_dynptr.c contains the following code:

  const size_t tcphdr_sz = sizeof(struct tcphdr);
  const size_t udphdr_sz = sizeof(struct udphdr);
  const size_t ethhdr_sz = sizeof(struct ethhdr);
  const size_t iphdr_sz = sizeof(struct iphdr);
  const size_t ipv6hdr_sz = sizeof(struct ipv6hdr);

  [...]

  static __always_inline int handle_ipv4(struct xdp_md *xdp, struct bpf_dynptr *xdp_ptr)
  {
	__u8 eth_buffer[ethhdr_sz + iphdr_sz + ethhdr_sz];
	__u8 iph_buffer_tcp[iphdr_sz + tcphdr_sz];
	__u8 iph_buffer_udp[iphdr_sz + udphdr_sz];
	[...]
  }

The eth_buffer, iph_buffer_tcp and other automatics are fixed size
only if the compiler optimizes away the constant global variables.
clang does this, but GCC does not, turning these automatics into
variable length arrays.

This patch removes the global variables and turns these values into
preprocessor constants.  This makes the selftest to build properly
with GCC.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: Yonghong Song <yhs@meta.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240123201729.16173-1-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>