[PATCH] Convert the VFS cache lock to an rmlock

I've just submitted a patch to Differential[1] for review that converts the
VFS cache to use an rmlock in place of the current rwlock.  My main
motivation for the change is to fix a priority inversion problem that I saw
recently.  A real-time priority thread attempted to acquire a write lock on
the VFS cache lock, but there was already a reader holding it.  The reader
was preempted by a normal priority thread, and my real-time thread was
starved.

[1] https://reviews.freebsd.org/D2051


I was worried about the performance implications of the change, as I wasn't
sure how common write operations on the VFS cache would be.  I did a -j12
buildworld/buildkernel test on a 12-core Haswell Xeon system, as I figured
that would be a reasonable stress test that simultaneously creates lots of
small files and reads a lot of files as well.  This actually wound up being
about a 10% performance *increase* (the units below are seconds of elapsed
time as measured by /usr/bin/time, so smaller is better):

$ ministat -C 1 orig.log rmlock.log
x orig.log
+ rmlock.log
+------------------------------------------------------------------------------+
|  +                                                                     x
    |
|++++                                            x                    x xxx
   |
| |A|
 |_________A___M____||
+------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   6       2710.31       2821.35       2816.75     2798.0617     43.324817
+   5       2488.25       2500.25       2498.04      2495.756     5.0494782
Difference at 95.0% confidence
        -302.306 +/- 44.4709
        -10.8041% +/- 1.58935%
        (Student's t, pooled s = 32.4674)

The one outlier in the rwlock case does confuse me a bit.  What I did was
booted a freshly-built image with the rmlock lock applied, did a git
checkout of head, and then did 5 builds in a row.  The git checkout should
have had the effect of priming the disk cache with the source files.  Then
I installed the stock head kernel, rebooted, and ran 5 more builds (and
then 1 more when I noticed the outlier).  The fast outlier was the *first*
run, which should have been running with a cold disk cache, so I really
don't know why it would be 90 seconds faster.  I do see that this run also
had about 500-600 fewer seconds spent in system time:

x orig.log
+------------------------------------------------------------------------------+
|
x             |
|x                                                        x   x
xx             |
|
|_________________________A__________M_____________||
+------------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x   6       3515.23       4121.84       4105.57       4001.71     239.61362

I'm not sure how much that I care, given that the rmlock is universally
faster (but maybe I should try the "cold boot" case anyway).

If anybody had any comments or further testing that they would like to see,
please let me know.