Cache bypass: Fix oops in bypass transition functions

Cache bypass: Fix oops in bypass transition functions

The FCSBypass flag might change between the time we check it before
entering afs_TransitionToCaching or afs_TransitionToBypass and when
we check it again within the functions.

Instead of panicing, just exit if someone beat us to it.  Also move
the checks within the write lock region to make sure the code
doesn't get run multiple times.

Reviewed-on: http://gerrit.openafs.org/3266
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit f2c6ae300a211460c24c87752cf9400b9e61ee1f)

Change-Id: I3b7ad3f9672975a87dbe337256d42166fc638b34
Reviewed-on: http://gerrit.openafs.org/3639
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>