perf mmap: Discard 'prev' in perf_mmap__read()

perf mmap: Discard 'prev' in perf_mmap__read()

The 'start' and 'prev' variables are duplicates in perf_mmap__read().

Use 'map->prev' to replace 'start' in perf_mmap__read_*().

Suggested-by: Wang Nan <[email protected]>
Signed-off-by: Kan Liang <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jin Yao <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>