Cache bypass: make readpage deal with reads at end of file

Cache bypass: make readpage deal with reads at end of file

When a file's size is an exact multiple of the page size, the vfs
will issue a readpage for an extra page at the end, for which there
is no data.  Deal with it here instead of letting it trickle down
to the background daemon, which will issue an unnecessary read to the
server, and maybe get confused because there is no data.

Reviewed-on: http://gerrit.openafs.org/3281
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 8ec31f26770ef1e85fb3a6005467f0e2d3ce1715)

Change-Id: Ib47a8cc62611c6e7c904898990b38ae48d8478e3
Reviewed-on: http://gerrit.openafs.org/3642
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>