arc_reclaim_thread running hot

[Eric van Gyzen]

This morning, I found the arc_reclaim_thread running hot on my laptop 
running 12.0-ALPHA5 r338572.

vfs.zfs.arc_max="4294967296"  <-- 4 GiB

last pid: 13288;  load averages:  1.32,  1.26,  1.16 

Mem: 456M Active, 3837M Inact, 743M Laundry, 2563M Wired, 167M Free
ARC: 1131M Total, 304M MFU, 145M MRU, 1344K Anon, 9116K Header, 671M Other
      89M Compressed, 361M Uncompressed, 4.03:1 Ratio

    22 root         -8    -      0   256K CPU2     2 309:20  99.75% 
zfskern{arc_reclaim_thread}

zfs_arc_meta_strategy is still the default of 1.

I sampled the thread's stacks with

for N in `jot 1000`; do procstat -kk 100101; done | grep 100101

and put the results here:

https://people.freebsd.org/~vangyzen/arc_reclaim_thread_stacks.txt

I'm happy to help debug this.  Just let me know what you need.

Eric