net: Use preallocated buffer for epoll.
net: Use preallocated buffer for epoll.

Reduces rpc test "mallocs per rpc round trip" by 4.

Gains ~8% on a localhost client/server RPC test.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4284056

dsriram/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,ssrl/go,dsriram/golang,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,apparentlymart/golang,ssrl/go,gnanderson/go,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,ssrl/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,gnanderson/go,ssrl/go