SUNRPC: pipefs per-net operations helper introduced

During per-net pipes creation and destruction we have to make sure, that pipefs
sb exists for the whole creation/destruction cycle. This is done by using
special mutex which controls pipefs sb reference on network namespace context.
Helper consists of two parts: first of them (rpc_get_dentry_net) searches for
dentry with specified name and returns with mutex taken on success. When pipe
creation or destructions is completed, caller should release this mutex by
rpc_put_dentry_net call.

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

net/sunrpc/netns.h

@@ -11,6 +11,7 @@ struct sunrpc_net {
 	struct cache_detail *ip_map_cache;
 
 	struct super_block *pipefs_sb;
+	struct mutex pipefs_sb_lock;
 };
 
 extern int sunrpc_net_id;