perf lock: Correct field name "flags"

perf lock: Correct field name "flags"

The tracepoint "lock:lock_acquire" contains field "flags" but not
"flag".  Current code wrongly retrieves value from field "flag" and it
always gets zero for the value, thus "perf lock" doesn't report the
correct result.

This patch replaces the field name "flag" with "flags", so can read out
the correct flags for locking.

Fixes: e4cef1f65061 ("perf lock: Fix state machine to recognize lock sequence")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20201104094229.17509-1-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>