tools/bpf: fix a selftest test_btf failure

tools/bpf: fix a selftest test_btf failure

Commit 9c651127445c ("selftests/btf: add initial BTF dedup tests")
added dedup tests in test_btf.c.
It broke the raw test:
 BTF raw test[71] (func proto (Bad arg name_off)):
    btf_raw_create:2905:FAIL Error getting string #65535, strs_cnt:1

The test itself encodes invalid func_proto parameter name
offset 0xffffFFFF as a negative test for the kernel.
The above commit changed the meaning of that offset and
resulted in a user space error.
  #define NAME_NTH(N) (0xffff0000 | N)
  #define IS_NAME_NTH(X) ((X & 0xffff0000) == 0xffff0000)
  #define GET_NAME_NTH_IDX(X) (X & 0x0000ffff)

Currently, the kernel permits maximum name offset 0xffff.
Set the test name off as 0x0fffFFFF to trigger the kernel
verification failure.

Cc: Andrii Nakryiko <andriin@fb.com>
Fixes: 9c651127445c ("selftests/btf: add initial BTF dedup tests")
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>