tested new mutex checking in sanoid snapshot thinning. added sleepymutex tool for future dev use in testing.

sleepymutex

@@ -0,0 +1,8 @@
+#!/bin/bash
+
+# this is just a cheap way to trigger mutex-based checks for process activity.
+#
+# ie ./sleepymutex zfs receive data/lolz if you want a mutex hanging around as long as necessary that will show up
+# to any routine that actively does something like "ps axo | grep 'zfs receive'" or whatever.
+
+sleep 99999