pipe: set file->private_data to ->i_pipe

simplify get_pipe_info(), while we are at it

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/coredump.c

@@ -409,9 +409,7 @@ static void coredump_finish(struct mm_struct *mm)
 
 static void wait_for_dump_helpers(struct file *file)
 {
-	struct pipe_inode_info *pipe;
-
-	pipe = file_inode(file)->i_pipe;
+	struct pipe_inode_info *pipe = file->private_data;
 
 	pipe_lock(pipe);
 	pipe->readers++;