lightnvm: implemented trace events for lightnvm mapping

LightNVM can re-map incoming requests, translating their logical
addresses into corresponding physical addresses - this mapping
step starts from 'nvm_map_rq'.
This patch implements a new tracing event class to capture the
remapping of requests and wraps 'nvm_map_rq' in two trace events.
Consequently, the overhead of the mapping code can be surmised by
examining the trace log, provided the tracers are enabled in
/sys/kernel/debug/tracing/events/nvm/nvm_rq_map_{begin,end}.