Add disk latency dtrace script to freenas-debug

Ticket:	#5466
(cherry picked from commit 003d32ce6d84b9476bc6353c3473f7e4fd3db8ac)
Do not unload dtrace

Ticket:	#5466
(cherry picked from commit 281317614dfd41abe7a06473e675501e7e5e77c5)