symlink /dev/net/tun -> ../tun

This is the expected location on Linux and this makes 'ip tuntap' work.

Before:
  vsoc_x86_64:/ # ip tuntap add dev tun0 mode tun
  open: No such file or directory
  vsoc_x86_64:/ # ip tuntap add dev tap0 mode tap
  open: No such file or directory
  vsoc_x86_64:/ # ip tuntap list

After:
  vsoc_x86_64:/ # ip tuntap add dev tun0 mode tun
  vsoc_x86_64:/ # ip tuntap add dev tap0 mode tap
  vsoc_x86_64:/ # ip tuntap list
  tap0: tap UNKNOWN_FLAGS:800
  tun0: tun UNKNOWN_FLAGS:800

  $ adbz shell ls -ldZ / /dev /dev/tun /dev/net /dev/net/tun
  drwxr-xr-x 25 root   root u:object_r:rootfs:s0         4096 2020-01-25 09:48 /
  drwxr-xr-x 21 root   root u:object_r:device:s0         1240 2020-01-25 09:48 /dev
  drwxr-xr-x  2 root   root u:object_r:device:s0           60 2020-01-25 09:48 /dev/net
  lrwxrwxrwx  1 root   root u:object_r:device:s0            6 2020-01-25 09:48 /dev/net/tun -> ../tun
  crw-rw----  1 system vpn  u:object_r:tun_device:s0  10, 200 2020-01-25 09:48 /dev/tun

Test: see above
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2aa215711454ce4f8a0ef1f34c17621629060fa1