xdr: fix two old FIXMEs related to signed/unsigned arithmetic

xdr: fix two old FIXMEs related to signed/unsigned arithmetic

It's implementation-defined whether the C '>>' operator, when
applied to a signed integer, is sign-extending or zero-filling.
If you want unsigned arithmetic, you have to ask for it explicitly.
One assumes the reason for the shift is to avoid overflow if the
returned size/count is later converted to a signed int, in which
case maybe it would be better to use INT_MAX here.  This is the
minimal change necessary for correctness.

Change-Id: I6e848110963b5e1832a11d052d84884f10903e2e
Reviewed-on: http://gerrit.openafs.org/7800
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>