The compression code was using isize to limit the amount of data it
sent through zlib.  But, it wasn't properly limiting the looping to
just the pages inside i_size.  The end result was trying to compress
too many pages, including those that had not been setup and properly locked
down.  This made the compression code oops while trying find_get_page on a
page that didn't exist.
Signed-off-by: Chris Mason <chris.mason@oracle.com>