procfs: new helper - PDE_DATA(inode)

The only part of proc_dir_entry the code outside of fs/proc
really cares about is PDE(inode)->data.  Provide a helper
for that; static inline for now, eventually will be moved
to fs/proc, along with the knowledge of struct proc_dir_entry
layout.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/block/pktcdvd.c b/drivers/block/pktcdvd.c
index 2e7de7a..e0588c6 100644
--- a/drivers/block/pktcdvd.c
+++ b/drivers/block/pktcdvd.c
@@ -2648,7 +2648,7 @@
 
 static int pkt_seq_open(struct inode *inode, struct file *file)
 {
-	return single_open(file, pkt_seq_show, PDE(inode)->data);
+	return single_open(file, pkt_seq_show, PDE_DATA(inode));
 }
 
 static const struct file_operations pkt_proc_fops = {