De-flakify sock_diag_test.

SockDestroyUdpTest.testClosesUdpSockets was failing a low
percentage of the time because of two bugs:

1. When creating a UDP socket pair, we were connecting one socket
   to the other, but the other socket to iself. This made it
   possible for sockets on ::ffff:127.0.0.1 and ::1 to be bound
   to the same port, and:
2. The code that attempts to get a diag_msg for a given socket
   did not consider the fact that socket dumps, unlike operations
   that operate on a specific socket with a cookie, do not use
   the IP addresses in idiag_src and idiag_dst. This made it
   so that attempting to close a socket on [::1:1234 might have
   instead closed a different socket on [::ffff:127.0.0.1]:1234.

Fix #1 by not doing the wrong thing, and fix #2 by using the
inode number to ensure that FindSockInfoFromFd returns the
requested socket.

Also remove FindSockInfoFromReq, since it was only used once and
was potentially misleading - while it only returns one socket, it
might not return the one the caller expects. This allows us to
tighten the code and ensure that operations that are supposed to
return one socket only ever return one socket.

Test: all_tests.sh passes on android-3.18.
Bug: 31119353
Change-Id: I5d65e5a30c37490db516b5c6e730f89b5fea1b27