jfs: microoptimize get_zeroed_page / virt_to_page

get_zeroed_page does alloc_page and returns page_address of the result;
subsequent virt_to_page will recover the page, but since the caller
needs both page and its page_address() anyway, why bother going through
that wrapper at all?

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>