SUNRPC: Add a field to track the number of kernel users of an rpc_pipe

This allows us to correctly deduce when we need to remove the pipe.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/rpc_pipe_fs.h

@@ -27,6 +27,7 @@ struct rpc_inode {
 	int pipelen;
 	int nreaders;
 	int nwriters;
+	int nkern_readwriters;
 	wait_queue_head_t waitq;
 #define RPC_PIPE_WAIT_FOR_OPEN	1
 	int flags;