Synchronously send packets over pipe link endpoint
Synchronously send packets over pipe link endpoint

Before this change, packets were delivered asynchronously to the remote
end of a pipe. This was to avoid a deadlock during link resolution where
the stack would attempt to double-lock a mutex (see removed comments in
the parent commit for details).

As of https://github.com/google/gvisor/commit/4943347137, we do not hold
locks while sending link resolution probes so the deadlock will no
longer occur.

PiperOrigin-RevId: 356066224

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor