bpftool: Fix a bug in subskeleton code generation

bpftool: Fix a bug in subskeleton code generation

Compiled with clang by adding LLVM=1 both kernel and selftests/bpf
build, I hit the following compilation error:

In file included from /.../tools/testing/selftests/bpf/prog_tests/subskeleton.c:6:
  ./test_subskeleton_lib.subskel.h:168:6: error: variable 'err' is used uninitialized whenever
      'if' condition is true [-Werror,-Wsometimes-uninitialized]
          if (!s->progs)
              ^~~~~~~~~
  ./test_subskeleton_lib.subskel.h:181:11: note: uninitialized use occurs here
          errno = -err;
                   ^~~
  ./test_subskeleton_lib.subskel.h:168:2: note: remove the 'if' if its condition is always false
          if (!s->progs)
          ^~~~~~~~~~~~~~

The compilation error is triggered by the following code
        ...
        int err;

        obj = (struct test_subskeleton_lib *)calloc(1, sizeof(*obj));
        if (!obj) {
                errno = ENOMEM;
                goto err;
        }
        ...

  err:
        test_subskeleton_lib__destroy(obj);
        errno = -err;
        ...
in test_subskeleton_lib__open(). The 'err' is not initialized, yet it
is used in 'errno = -err' later.

The fix is to remove 'errno = -err' since errno has been set properly
in all incoming branches.

Fixes: 00389c58ffe9 ("bpftool: Add support for subskeletons")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220320032009.3106133-1-yhs@fb.com