SUNRPC: replace inode lock with pipe lock for RPC PipeFS operations

Currenly, inode i_lock is used to provide concurrent access to SUNPRC PipeFS
pipes. It looks redundant, since now other use of inode is present in most of
these places and thus can be easely replaced, which will allow to remove most
of inode references from PipeFS code. This is a first step towards to removing
PipeFS inode references from kernel code other than PipeFS itself.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/rpc_pipe_fs.h

@@ -35,6 +35,7 @@ struct rpc_inode {
 	int flags;
 	struct delayed_work queue_timeout;
 	const struct rpc_pipe_ops *ops;
+	spinlock_t lock;
 };
 
 static inline struct rpc_inode *