SUNRPC: put pipefs superblock link on network namespace

We have modules (like, pNFS blocklayout module) which creates pipes on
rpc_pipefs. Thus we need per-net operations for them. To make it possible we
require appropriate super block. So we have to put sb link on network namespace
context. Note, that it's not strongly required to create pipes in per-net
operations. IOW, if pipefs wasn't mounted yet, that no sb link reference will
present on network namespace and in this case we need just need to pass through
pipe creation. Pipe dentry will be created during pipefs mount notification.

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

net/sunrpc/netns.h

@@ -9,6 +9,8 @@ struct cache_detail;
 struct sunrpc_net {
 	struct proc_dir_entry *proc_net_rpc;
 	struct cache_detail *ip_map_cache;
+
+	struct super_block *pipefs_sb;
 };
 
 extern int sunrpc_net_id;

net/sunrpc/rpc_pipe.c

@@ -1029,6 +1029,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 	struct inode *inode;
 	struct dentry *root;
 	struct net *net = data;
+	struct sunrpc_net *sn = net_generic(net, sunrpc_net_id);
 	int err;
 
 	sb->s_blocksize = PAGE_CACHE_SIZE;
@@ -1053,6 +1054,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto err_depopulate;
 	sb->s_fs_info = get_net(net);
+	sn->pipefs_sb = sb;
 	return 0;
 
 err_depopulate:
@@ -1073,7 +1075,9 @@ rpc_mount(struct file_system_type *fs_type,
 void rpc_kill_sb(struct super_block *sb)
 {
 	struct net *net = sb->s_fs_info;
+	struct sunrpc_net *sn = net_generic(net, sunrpc_net_id);
 
+	sn->pipefs_sb = NULL;
 	put_net(net);
 	blocking_notifier_call_chain(&rpc_pipefs_notifier_list,
 					   RPC_PIPEFS_UMOUNT,