viced: Don't store rc from IH_DEC on error path

If CopyOnWrite has already failed, and is taking the volume offline
for salvage we can't (and don't) do anything with further errors from
IH_DEC. Instead of assigning these to 'rc', just ignore them.

Caught by clang-analyzer

Change-Id: I7bcd82f124740e3ca3d11cb2b079aa7ea5751cf9
Reviewed-on: http://gerrit.openafs.org/9239
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>