selftest: af_unix: Extend recv() timeout in so_peek_off.c.

selftest: af_unix: Extend recv() timeout in so_peek_off.c.

so_peek_off.c is reported to be flaky on NIPA:

  # # so_peek_off.c:149:two_chunks_overlap_blocking:Expected -1 (-1) != bytes (-1)
  # # two_chunks_overlap_blocking: Test terminated by assertion
  # #          FAIL  so_peek_off.stream.two_chunks_overlap_blocking

The test fork()s a child process to send() data after 1ms to
wake up the parent process being blocked (up to 3ms) on recv().

But, from the log, the parent woke up after 3ms timeout, so it
could be too short when the host is overloaded.

Let's extend it to 5s.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20251124070722.1e828c53@kernel.org/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251124212805.486235-3-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>