Upstream: dmu_zfetch_stream_fini leaks refcount

dmu_zfetch_stream_fini() is missing calls to destroy the refcounts,
leaking them and the mutex inside.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12294