Linux: Don't read pages beyond the cache eof

Linux: Don't read pages beyond the cache eof

If we attempt to read past the end of the current cache file (for
example, when we're extending the file with ftruncate), don't force
the backend filesystem to populate that page with non-existent data.

This will hopefully fix a bus error when using tmpfs as a backing
cache.

FIXES 128452

Change-Id: I087aa1587885e97493130e5d05db6a1ed961181a
Reviewed-on: http://gerrit.openafs.org/4562
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>