Don't use entry after free in the "already in lists" case

Return with 0 as it isn't an error.

MFC after: 1 week
CID: 1006085
Reported by: Coverity
Obtained from: Isilon OneFS (part of r493633)
Submitted by: Thor Steingrimsson <thor.steingrimsson@isilon.com>
Sponsored by: EMC / Isilon Storage Division