ssh: fix deadlock
ssh: fix deadlock

The code was taking locks in the wrong order.

Fixes issue 3570.

R=fullung
CC=golang-dev
http://codereview.appspot.com/6123058

cmars/sshlurp-go.crypto,xman1101/terminal,tonnerre/golang-go.crypto,cmars/go.crypto,drhax9908/go.crypto,xman1101/terminal,drhax9908/go.crypto,isislovecruft/go.crypto,polydawn-ports/go.crypto,polydawn-ports/go.crypto,dizzyd/go.crypto,dizzyd/go.crypto,isislovecruft/go.crypto,cmars/sshlurp-go.crypto,cmars/go.crypto,tonnerre/golang-go.crypto