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.

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>
(cherry picked from commit 5b67620c7b3ad224ea53075e37ecf1f7e6a7c51a)

Change-Id: Iccb8ca2625211b94b105c6eb60764f4a064b345a
Reviewed-on: http://gerrit.openafs.org/10518
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>