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

Change-Id: Iaecd68764f4b071d6b8c838362e7e657578d9b2f
Reviewed-on: http://gerrit.openafs.org/10876
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>