net: avoid allocation in setAddr
net: avoid allocation in setAddr

setAddr was showing up in profiles due to string concatenation construction the os.File name field. netFD.sysfile's Name() is never used, except in dup() so I believe it is safe to avoid this allocation.

R=mikioh.mikioh, rsc
CC=golang-dev
http://codereview.appspot.com/6742058
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go