Linux: Fix tmpfs cache support

As of kernel 3.1, tmpfs no longer has a readpage() operation in its
address space operations.  Some of the cache manager code relies on
this, causing an oops if tmpfs is used as backing store for the
cache.

As a minimal fix, detect that there is no readpage() and disable
the optimizations that depend on it.

Change-Id: I6e2236cd53f893d77a8d6568eae7e200ad8d1ac0
Reviewed-on: http://gerrit.openafs.org/9949
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>