datapath: Reduce loop limit by one to 4.

datapath: Reduce loop limit by one to 4.

We currently allow five trips through the kernel datapath
before dropping the packet to protect the stack.  However, there
have been a few reports recently involving tunneling that this is
still too much.  Although it's not a complete solution, this reduces
the limit by one to balance safety in common situations with
flexibility.

Bug #15477

Reported-by: Paul Ingram <paul@nicira.com>
Reported-by: 謝秉融 <faithfulman@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>