SUNRPC: fix NFS client over TCP hangs due to packet loss (Bug 16494)

When reusing a TCP connection, ensure that it's aborted if a previous
shutdown attempt has been made on that connection so that the RPC over
TCP recovery mechanism succeeds.

Signed-off-by: Andy Chittenden <andyc.bluearc@gmail.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>