samples/bpf: fix sockex2 example

since llvm commit "Do not expand UNDEF SDNode during insn selection lowering"
llvm will generate code that uses uninitialized registers for cases
where C code is actually uses uninitialized data.
So this sockex2 example is technically broken.
Fix it by initializing on the stack variable fully.
Also increase verifier buffer limit, since verifier output
may not fit in 64k for this sockex2 code depending on llvm version.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>