selftests/bpf: Fix RELEASE build failure with gcc14

selftests/bpf: Fix RELEASE build failure with gcc14

With gcc14, when building with RELEASE=1, I hit four below compilation
failure:

Error 1:
  In file included from test_loader.c:6:
  test_loader.c: In function ‘run_subtest’: test_progs.h:194:17:
      error: ‘retval’ may be used uninitialized in this function
   [-Werror=maybe-uninitialized]
    194 |                 fprintf(stdout, ##format);           \
        |                 ^~~~~~~
  test_loader.c:958:13: note: ‘retval’ was declared here
    958 |         int retval, err, i;
        |             ^~~~~~

  The uninitialized var 'retval' actually could cause incorrect result.

Error 2:
  In function ‘test_fd_array_cnt’:
  prog_tests/fd_array.c:71:14: error: ‘btf_id’ may be used uninitialized in this
      function [-Werror=maybe-uninitialized]
     71 |         fd = bpf_btf_get_fd_by_id(id);
        |              ^~~~~~~~~~~~~~~~~~~~~~~~
  prog_tests/fd_array.c:302:15: note: ‘btf_id’ was declared here
    302 |         __u32 btf_id;
        |               ^~~~~~

  Changing ASSERT_GE to ASSERT_EQ can fix the compilation error. Otherwise,
  there is no functionality change.

Error 3:
  prog_tests/tailcalls.c: In function ‘test_tailcall_hierarchy_count’:
  prog_tests/tailcalls.c:1402:23: error: ‘fentry_data_fd’ may be used uninitialized
      in this function [-Werror=maybe-uninitialized]
     1402 |                 err = bpf_map_lookup_elem(fentry_data_fd, &i, &val);
          |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

  The code is correct. The change intends to silence gcc errors.

Error 4: (this error only happens on arm64)
  In file included from prog_tests/log_buf.c:4:
  prog_tests/log_buf.c: In function ‘bpf_prog_load_log_buf’:
  ./test_progs.h:390:22: error: ‘log_buf’ may be used uninitialized [-Werror=maybe-uninitialized]
    390 |         int ___err = libbpf_get_error(___res);             \
        |                      ^~~~~~~~~~~~~~~~~~~~~~~~
  prog_tests/log_buf.c:158:14: note: in expansion of macro ‘ASSERT_OK_PTR’
    158 |         if (!ASSERT_OK_PTR(log_buf, "log_buf_alloc"))
        |              ^~~~~~~~~~~~~
  In file included from selftests/bpf/tools/include/bpf/bpf.h:32,
                 from ./test_progs.h:36:
  selftests/bpf/tools/include/bpf/libbpf_legacy.h:113:17:
    note: by argument 1 of type ‘const void *’ to ‘libbpf_get_error’ declared here
    113 | LIBBPF_API long libbpf_get_error(const void *ptr);
        |                 ^~~~~~~~~~~~~~~~

  Adding a pragma to disable maybe-uninitialized fixed the issue.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250617044956.2686668-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>