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.

Reviewed-on: http://gerrit.openafs.org/9949
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3767cf8bf420bd59c3426a4287259b1e97178e96)

Change-Id: I4fa0f2449eabf4dc82e5450c0b378b9ebe49d848
Reviewed-on: http://gerrit.openafs.org/9950
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>