In preparation for removing kmap from directory handling, use
offset_in_page() to calculate 'from'.  Matches ext2.

Link: https://lkml.kernel.org/r/20231127143036.2425-5-konishi.ryusuke@gmail.comSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>