LU-15252 mdt: reduce contention at mdt_lsom_update

LU-15252 mdt: reduce contention at mdt_lsom_update

mot_som_mutex serialize all close requests with lsom updates for
a same mdt_object. For a massive open/read/close single shared
file load, it leads to high load avarage cause many threads sleep
on mutex.
This patch introduces a cached lsom size, and uses a mutex at update
part only. Close requests with lsom size less or equal to cached size
would not take a mutex at all.

Test results MPI open/flock/funlock/close SSF
10 iterations 10 node 100 thread each, 1000 file ops per thread
close time secs master patch MDT load avarage master patch
avg             0.142  0.086                  47.05  38.89
max             0.164  0.129                  49.39  44.77
min             0.097  0.041                  44.44  34.7

Signed-off-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I807b468b128295df9391b0467e74d4f10240662e
Reviewed-on: https://review.whamcloud.com/45709
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>