ext2: use offset_in_page() instead of open-coding it as subtraction

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>