datapath: return -EEXIST if inet6_add_protocol fails

datapath: return -EEXIST if inet6_add_protocol fails

Our code to determine whether receive functionality will work with
ip6 gre depends on the return of -EEXIST but inet6_add_protocol()
returns a -1 on failure to grab the pointer via a cmpxchg op.  Just
set the error return to -EEXIST to help out the vport init function.

Reviewed-by: Yifeng Sun <pkusunyifeng@gmail.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2019-January/048090.html
Reported-by: Ken Ajiro <ken-ajiro@xr.jp.nec.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>