LU-18576 obdclass: do not update llog's timestamp each time

LU-18576 obdclass: do not update llog's timestamp each time

updating llog's timestamp on every record doesn't makes sense as
we save seconds and introduces performance penalty (upto 15% in
microbenchmark). it should be enough to update it every second.

the results, in msec per record inserted:

32K records into a plain llog:
mtime update:	yes	no
1 thread	2421	2137
4 threads	1941	1687

120K records into a catalog:
mtime update	yes	no
1 thread	3007	2765
4 threads	2012	1744

Fixes: e33d196b93 ("LU-17309 llog: set timestamp on llog objects at creation time")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I0178ba7071ce39668550b4faa5e1e86397c8a89d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57510
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: kg.xu <squalfof@gmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>