OpenZFS 7659 - Missing thread_exit() in dmu_send.c

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: https://github.com/openzfs/openzfs/commit/a569268
Closes #5603