selftests/bpf: fix a test for snprintf() overflow

The snprintf() function returns the number of bytes which *would*
have been copied if there were space.  In other words, it can be
> sizeof(pin_path).

Fixes: c0fa1b6c ("bpf: btf: Add BTF tests")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/YtZ+aD/tZMkgOUw+@kili


Signed-off-by: Alexei Starovoitov <ast@kernel.org>