Use page_offset() on Linux

Use page_offset() on Linux

Since time began, Linux has provided page_offset() to turn a
page index into an offset into the corresponding file. Make us use
this everywhere we calculate a page offset, rather than doing our
own maths.

Reviewed-on: http://gerrit.openafs.org/581
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>