Fix sysfs mount in net_test.sh.

The new net_test.sh used by build_rootfs.sh used a broken filesystem
type for sysfs. This meant that sysfs was not mounted. This did not
actually break anything because sysfs does not appear to be used by the
tests.

Bug: 79694505
Test: run_net_test.sh all_tests.sh
Change-Id: I781902d5561f5d7a9815cd886f30bf7d27bf996e
Signed-off-by: Alistair Strachan <astrachan@google.com>