SUNRPC: Ensure we always bump the backlog queue in xprt_free_slot

Whenever we free a slot, we know that the resulting xprt->num_reqs will
be less than xprt->max_reqs, so we know that we can release at least one
backlogged rpc_task.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org [>=3.1]