viced: CopyOnWrite2 shouldn't return undefined val

viced: CopyOnWrite2 shouldn't return undefined val

Unless CopyOnWrite2 encounters the condition where
(rdlen != length) || (wrlen !=length) it will never set a value
for the 'rc' return code, and so return with an undefined value.

Looking at the code, it looks like rc should be 0 in all other
situations, so this patch provides a default value.

Caught by clang-analyzer

Change-Id: If3e528f3d66abb3dde6da2304084f7698c05b863
Reviewed-on: http://gerrit.openafs.org/2141
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>