Initialize first_serial to 0 in dozonetransfer(..) to fix -Wuninitialized
warning

MFC after: 3 days
X-MFC with: r302779
Pointyhat to: des
PR: 209177
Reported by: Jenkins (sparc64 job), gcc 4.2.1, gcc 5.3.0
Sponsored by: EMC / Isilon Storage Division