scripts/runqemu-ifup: Ensure netmask is set correctly
Without this the command will add a route for the subnet 192.168.7.0 which
means multiple qemu instances can't operate correctly since all but the last
one will be masked out.
(From OE-Core rev: 9e00d6b343120496ec0dd72240c7b04e0a8b7eaa)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>