volser: Remove masked assignment in vol_split

The declaration (and assignment) to size at the outermost scope
in the copyVnodes function is masked by a later declaration at a
inner scope.

Just remove the outer declaration to clarify what's happening.

Caught by clang-analyzer

Change-Id: If23bdcf8a4c2d23d6224762fecce35be9cf15873
Reviewed-on: http://gerrit.openafs.org/9168
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>