libafs: Get rx conn ref with afs conn ref

libafs: Get rx conn ref with afs conn ref

When we get a reference to an afs_conn with afs_Conn and its variants,
we assume we can use the tc->id rx connection without holding any
locks. However, if tc->forceConnectFS gets set, the tc->id connection
can be destroyed and recreated out from under us. So, to avoid using a
possibly freed rx connection, grab a reference to the rx connection at
the same time as we grab a reference to the afs conn. And also put
back the same reference with afs_PutConn.

Change-Id: I442886ee89c6f3fa609e47261317e647e124eecc
Reviewed-on: http://gerrit.openafs.org/4625
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>