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

tools/testing/selftests/bpf/progs/bpf_iter_unix.c

@@ -63,7 +63,7 @@ int dump_unix(struct bpf_iter__unix *ctx)
 			BPF_SEQ_PRINTF(seq, " @");
 
 			for (i = 1; i < len; i++) {
-				/* unix_mkname() tests this upper bound. */
+				/* unix_validate_addr() tests this upper bound. */
 				if (i >= sizeof(struct sockaddr_un))
 					break;