selftest/bpf: Fix a stale comment.

The commit b8a58aa6 ("af_unix: Cut unix_validate_addr() out of
unix_mkname().") moved the bound test part into unix_validate_addr().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/r/20220113002849.4384-6-kuniyu@amazon.co.jp


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