OpenZFS 7659 - Missing thread_exit() in dmu_send.c

Two threads send_traverse_thread() and receive_writer_thread() should
end with thread_exit();

Mostly a cosmetic issue under IllumOS.

Authored by: Jorgen Lundman <lundman@lundman.net>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7659
OpenZFS-commit: openzfs/openzfs@a569268
Closes #5603

module/zfs/dmu_send.c

@@ -613,6 +613,7 @@ send_traverse_thread(void *arg)
 	data->eos_marker = B_TRUE;
 	bqueue_enqueue(&st_arg->q, data, 1);
 	spl_fstrans_unmark(cookie);
+	thread_exit();
 }
 
 /*
@@ -2874,6 +2875,7 @@ receive_writer_thread(void *arg)
 	cv_signal(&rwa->cv);
 	mutex_exit(&rwa->mutex);
 	spl_fstrans_unmark(cookie);
+	thread_exit();
 }
 
 static int