net/templates/run.sh: tweak masquerade

Replace the old check with an exact check for the given rule. The old
check doesn't work on my system if docker is running and sets up other
masquerading rules.

Change -A to -I, so it is less likely to be affected by whatever other
rules may already be above in the chain.

Change-Id: I30c22fe4e7683611e8dada329dbf7d51f78b6016
1 file changed