viced: Check vnode length on read and write

viced: Check vnode length on read and write

When reading or writing a file vnode, check that the length of the
vnode in the vnode index matches the size of the on-disk file
containing the data for the file. If it does not match, take the
volume offline (and for DAFS, demand-salvage it).

Reviewed-on: http://gerrit.openafs.org/4121
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit aadf69eabb1962496fa93745ab560a5b48cacd61)

Change-Id: I324dc79a24fc8c199825ffefcc1dd2b0097942a1
Reviewed-on: http://gerrit.openafs.org/5768
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>