LINUX: Define zero_user_segment

LINUX: Define zero_user_segment

When the kernel does not have the zero_user_segments function, we
define it ourselves. Also define the zero_user_segment function, since
we use it, and a kernel lacking zero_user_segments will also lack
zero_user_segment.

Change-Id: I5c786d2af27227793a7a2768d1af76ccd2d6a415
Reviewed-on: http://gerrit.openafs.org/3432
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>