Fix bug in RstCausesPollHUP.
Fix bug in RstCausesPollHUP.

The test is checking the wrong poll_fd for POLLHUP. The only
reason it passed till now was because it was also checking
for POLLIN which was always true on the other fd from the
previous poll!

PiperOrigin-RevId: 270780401

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