SUNRPC: Add a transport callback to handle dequeuing of an RPC request

Add a transport level callback to allow it to handle the consequences of
dequeuing the request that was in the process of being transmitted.
For something like a TCP connection, we may need to disconnect if the
request was partially transmitted.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>