Avoid Linux hung task message in ZTHR

Use an interruptible to avoid Linux hung task message in
ZTHR and to prevent inflating the load average.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #7440 
Closes #7441

module/zfs/zthr.c

@@ -189,7 +189,7 @@ zthr_procedure(void *arg)
 			mutex_enter(&t->zthr_lock);
 		} else {
 			/* go to sleep */
-			cv_wait(&t->zthr_cv, &t->zthr_lock);
+			cv_wait_sig(&t->zthr_cv, &t->zthr_lock);
 		}
 	}
 	mutex_exit(&t->zthr_lock);