odp-util: Do not set port mask of non-IP packets

In the case that an flow for an IP packet has an mpls_push action applied
the L3 and L4 portions of the flow will be cleared in flow_push_mpls().

Without this change commit_set_port_action() will set the tp_src and tp_dst
mask for the flow to all-ones because the base and flow port values no
longer match. Even though there will be no corresponding set action for the
ports; because the flow is no longer IP.

In this case where nw_proto is not part of the match this manifests
in a problem because the kernel datapath rejects flows whose masks
have non-zero values for tp_src or dp_dst if the nw_proto mask is
not all-ones.

This patch resolves this problem by having commit_set_port_action() return
without doing anything if flow->nw_proto is zero. The same logic is present
in commit_set_nw_action().

Also enhance one of the MPLS tests to exercise this logic.  The enhanced
tests inputs a UDP packet with non-zero ports rather than an IP packet with
zeroed ports: zeroed ports cause commit_set_port_action() always return
without doing anything..

Commit 691d39b ("upcall: Remove redundant xlate_actions_for_side_effects().")
causes xlate_in_init() to be called for every packet that has an upcall.
This has the effect of indirectly calling commit_set_port_action() when
translating a controller action which may not have previously been the case
depending on the flow.

The result is that the behaviour described in the changelog above can be
exercised via a minor enhancement to one of the existing MPLS tests. This
illustrates that the problem exists for the user-space datapath whereas I
had previously incorrectly assumed it only manifested when using the kernel
datapath because I had only observed it there.

Signed-off-by: Simon Horman <horms@verge.net.au>
Acked-by: Jarno Rajahalme <jrajahame@nicira.com>
INSTALL: Note about compiler atomics support.

OVS is slow when compiled with pthreads atomics.  Add a generic note
in INSTALL, with a reference to lib/ovs-atomic.h, where a new comment
provides additional detail.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>