reiserfs: Don't access the proc_dir_entry in r_open(), r_start() r_show()

Don't access the proc_dir_entry in ReiserFS's r_open(), r_start() r_show()
procfs interface functions.

ReiserFS stores the ->show() method pointer in PDE->data and the super_block
pointer in PDE->parent->data.  This isn't changing.

Currently, ReiserFS passes the PDE pointer into seq_file::private from
r_open() so that r_start() and r_show() can then access it.  Instead, use
seq_open_private() to allocate a two-pointer struct that's passed through
seq_file::private and put the ->show() method and the sb pointers in there.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: reiserfs-devel@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>