Cache bypass: Only compile bypass code for the Linux kernel

Cache bypass: Only compile bypass code for the Linux kernel

Only compile the afs_bypasscache.c code if AFS_LINUX24_ENV is set,
since it's currently the only case where the code is actually used.
Only sections that caused problems for UKERNEL were previously
ifdef'ed.

Besides making the code cleaner, the main effect of this change
is to prevent compiling most of the bypass code for UKERNEL where
it isn't currently used.

If support for bypass is added for other platforms eventually, the
code here would need to be reworked anyway, ideally abstracting out
and moving any OS specific bits into the platform subdirectories.

Reviewed-on: http://gerrit.openafs.org/3374
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 46a6d6129d80c4027d8a9f0e1d06b32cc12d6367)

Change-Id: I042a30ac2a5e8156a233a0ff0f46aa5cd4fcd675
Reviewed-on: http://gerrit.openafs.org/3646
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>