SUNRPC: xprt_complete_bc_request must also decrement the free slot count

Calling xprt_complete_bc_request() effectively causes the slot to be allocated,
so it needs to decrement the backchannel free slot count as well.

Fixes: 0d2a970d ("SUNRPC: Fix a backchannel race")
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/backchannel_rqst.c

@@ -336,7 +336,7 @@ void xprt_complete_bc_request(struct rpc_rqst *req, uint32_t copied)
 
 	spin_lock(&xprt->bc_pa_lock);
 	list_del(&req->rq_bc_pa_list);
-	xprt->bc_alloc_count--;
+	xprt_dec_alloc_count(xprt, 1);
 	spin_unlock(&xprt->bc_pa_lock);
 
 	req->rq_private_buf.len = copied;