From: Neil Brown <neilb@cse.unsw.edu.au>

If a single readdir entry needs to be split over two pages in the reply, we
first encode it into a new page, and then copy the bits into place.  When
we do this relocation, we have to modify the "offset" pointer to be either
in the first or the second page, as appropriate.

If the pointer should be at the start of the second page, it is currently
put past the end of the first page.

Note that as the offset and whole response is known to be 4byte-aligned,
the offset pointer will never be split over two pages.