sunrpc: make xprt->swapper an atomic_t

Split xs_swapper into enable/disable functions and eliminate the
"enable" flag.

Currently, it's racy if you have multiple swapon/swapoff operations
running in parallel over the same xprt. Also fix it so that we only
set it to a memalloc socket on a 0->1 transition and only clear it
on a 1->0 transition.

Cc: Mel Gorman <mgorman@suse.de>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 0ba6515..801044a 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -2495,7 +2495,7 @@
 			goto retry;
 		}
 
-		ret = xs_swapper(xprt, 1);
+		ret = xs_swapper_enable(xprt);
 		xprt_put(xprt);
 	}
 	return ret;
@@ -2522,7 +2522,7 @@
 			goto retry;
 		}
 
-		xs_swapper(xprt, 0);
+		xs_swapper_disable(xprt);
 		xprt_put(xprt);
 	}
 }