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

Reviewed-on: http://gerrit.openafs.org/4562
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 79d5b5cce65b10134004c4cb2b7b34ac509cba6a)

Change-Id: Id9956be824a4c4d8db7deb65403f4d9740758e42
Reviewed-on: http://gerrit.openafs.org/4600
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>