run_net_test.sh: fix UML --readonly flag

Without this you end up with r=... instead of ubdar=... and:

  Failed to initialize ubd device 0 :Couldn't determine size of device's file
  VFS: Cannot open root device "98:0" or unknown-block(98,0): error -6
  Please append a correct "root=" boot option; here are the available partitions:
  Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(98,0)
  Aborted

Change-Id: If0d53a8560b8dca8908f30794550b7a1360c85a7