ext2_get_page(): saner type

We need to pass to caller both the page reference and pointer to the
first byte in the now-mapped page.  The former always has the same type,
the latter varies from caller to caller.  So make it
	void *ext2_get_page(...., struct page **page)
rather than
	struct page *ext2_get_page(..., void **page_addr)
and avoid the casts...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Tested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>