perf lock contention: Account contending locks too

Currently it accounts the contention using delta between timestamps in
lock:contention_begin and lock:contention_end tracepoints.  But it means
the lock should see the both events during the monitoring period.

Actually there are 4 cases that happen with the monitoring:

                monitoring period
            /                       \
            |                       |
 1:  B------+-----------------------+--------E
 2:    B----+-------------E         |
 3:         |           B-----------+----E
 4:         |     B-------------E   |
            |                       |
            t0                      t1

where B and E mean contention BEGIN and END, respectively.  So it only
accounts the case 4 for now.  It seems there's no way to handle the case
1.  The case 2 might be handled if it saved the timestamp (t0), but it
lacks the information from the B notably the flags which shows the lock
types.  Also it could be a nested lock which it currently ignores.  So
I think we should ignore the case 2.

However we can handle the case 3 if we save the timestamp (t1) at the
end of the period.  And then it can iterate the map entries in the
userspace and update the lock stat accordinly.

Signed-off-by: Namhyung Kim <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Reviwed-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Song Liu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]

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