selftests/bpf: Test concurrent task local data key creation

selftests/bpf: Test concurrent task local data key creation

Test thread-safety of tld_create_key(). Since tld_create_key() does
not rely on locks but memory barriers and atomic operations to protect
the shared metadata, the thread-safety of the function is non-trivial.
Make sure concurrent tld_key_create(), both valid and invalid, can not
race and corrupt metatada, which may leads to TLDs not being thread-
specific or duplicate TLDs with the same name.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20250730185903.3574598-5-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>