Refactor the cache bypass determination code

Refactor the cache bypass determination code

The code which determines whether the cache can be bypassed occurs in
multiple locations in this file. Make a single static inline function
that can do this, and use it.

Reviewed-on: http://gerrit.openafs.org/489
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>