Have ifmp_ring_enqueue() abdicate instead of switch to a consumer

[shurd]

Summary

Move TX out of the enqueue() path. As a result, we need
to have ifmp_ring_check_drainage() pick up from the abdicate state.

We also need to either enqueue the TX task, or check drainage
after calling ifmp_ring_enqueue() to ensure it's sent.

This change results in a 30% small packet forwarding improvement.

Test Plan

Check for performance regressions when not forwarding
traffic. If regressions are present, we may need an "abdicate"
flag to selectively enable this behaviour.

[shurd]

Add Olivier.

[olivier]

Yeah!!! You've fixed the problem.

x FreeBSD 11.1-RELEASE: inet4 packets-per-second
+ FreeBSD 12-HEAD r323724: inet4 pps
* FreeBSD 12-HEAD r323724 with D12439: inet4 pps
+--------------------------------------------------------------------------+
|++                                                         *   *        x |
|++                                                         ********   x xx|
|                                                                       |A||
|A|                                                                        |
|                                                            |_A__|        |
+--------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   5        730075        738806        734556      734964.2      3292.366
+  10        495853        499834        496560      497141.3     1415.9471
Difference at 95.0% confidence
        -237823 +/- 2571.21
        -32.3584% +/- 0.274786%
        (Student's t, pooled s = 2173.31)
*  10        691125        714426      703487.5      703086.4     8356.6996
Difference at 95.0% confidence
        -31877.8 +/- 8505.21
        -4.33733% +/- 1.15405%
        (Student's t, pooled s = 7189.03)

And there is even a big improvement regarding inet6:

x FreeBSD 11.1-RELEASE: inet6 packets-per-second
+ FreeBSD 12-HEAD r323724: inet6 pps
* FreeBSD 12-HEAD r323724 with D12439: inet6 pps
+--------------------------------------------------------------------------+
|+                                                                       * |
|+                                                                       * |
|+                                                                       * |
|+                                                        x              * |
|+                                                        x              **|
|++                                                     x xx           ****|
|                                                        |A|               |
|A                                                                         |
|                                                                       |A||
+--------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   5        702120        712761        709023      708479.5     3919.0839
+  10        490039      492940.5      490692.5        490898     813.06116
Difference at 95.0% confidence
        -217582 +/- 2693.58
        -30.7111% +/- 0.275736%
        (Student's t, pooled s = 2276.75)
*  10      757768.5        769699      767084.5      766178.4     3437.8562
Difference at 95.0% confidence
        57698.9 +/- 4250.57
        8.14405% +/- 0.618294%
        (Student's t, pooled s = 3592.8)