ext2_put_page(): accept any pointer within the page

eliminates the need to keep the pointer to the first byte within
the page if we are guaranteed to have pointers to some byte
in the same page at hand.

Don't backport without commit 88d7b120 ("highmem: round down the
address passed to kunmap_flush_on_unmap()").

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>