Linux: Fix build for older kernels w/o bool

Commit b7f4f2023b2b3e1aac46715176940fb50cc75265 broke builds against
older kernels which don't have bool defined in linux/types.h . Fix
this by using unsigned char instead of bool for the static inline
functions.

Change-Id: Icbb82446ef66edd2650f33135ed6ccd2b8a920b2
Reviewed-on: http://gerrit.openafs.org/10483
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>