generic/176 test for btrfs from xfstests will hang

http://10.246.72.4/test-results/4.15.0-34.37~16.04.1-lowlatency/rizzo__4.15.0-34.37~16.04.1__2018-08-31_06-02-00/xfstests/results/xfstests.btrfs/debug/xfstests.btrfs.DEBUG.html

The xfstest generic/176 for btrfs will hang with 4.15.0-34.37~16.04.1 on node rizzo / starmie.
You can see the test will stop after generic/175, and timeouted:
494. 09/05 17:44:45 DEBUG| utils:0153| [stdout] generic/174 3s
495. 09/05 17:47:34 DEBUG| utils:0153| [stdout] generic/175 168s
496.

For other kernels, most of them will hang with the ext4 or the xfs filesystem test with xfstests. It's hard for them to reach the btrfs test.