LU-13309 osd: use per-cpu counters for brw_stats

LU-13309 osd: use per-cpu counters for brw_stats

Based on perf reports, oh_lock is highly contended
when running IOR with NVMe storage, so we need to
move to per-cpu counters.

struct brw_stats becomes larger: from 3872 to 18208 bytes.
Also, 4 bytes are allocated per each cpu for every counter.
With an 8-cpu system and 32 4-byte per-cpu counters,
there are 448 per-cpu counters or 1792 bytes per-cpu.
These counters will either reuse already
allocated per-cpu pages or allocate a new page on each cpu
(8 pages total).

Change-Id: I24536a0138067fb868aaf962d9321dea7566d13f
Signed-off-by: Andrew Perepechko <andrew.perepechko@hpe.com>
HPE-bug-id: LUS-8007, LUS-8185
Reviewed-on: https://review.whamcloud.com/37915
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>