volser: use also vn_length_hi in dump size calculation

Only the low order 32 bits of the file length were used.
Now using macro VNDISK_GET_LEN instead of direct FillInt64.

FIXES 131819

Reviewed-on: http://gerrit.openafs.org/10876
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 705f3ee384814bc082817267a2658bd5c918550c)

Change-Id: I1816e4b7c5fe1166ac81ff53235f9f676f4d234f
Reviewed-on: http://gerrit.openafs.org/10933
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>