zisofs: fix readpage() outside i_size

commit: 08ca0db8

A read request outside i_size will be handled in do_generic_file_read().  So
we just return 0 to avoid getting -EIO as normal reading, let
do_generic_file_read do the rest.

At the same time we need unlock the page to avoid system stuck.

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=10227



Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Acked-by: Jan Kara <jack@suse.cz>
Report-by: Christian Perle <chris@linuxinfotag.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>