os/bluestore: BlueFS: harmonize log read and writes modes

os/bluestore: BlueFS: harmonize log read and writes modes

BlueFS log has always been written in non-buffered mode.
Reading of it depends on bluefs_buffered_io option.
It is strongly suspected that this causes some wierd problems.

Possibly fixes: https://tracker.ceph.com/issues/54019

It is targetted directly to pacific.
Ultimately same fix will go to all versions.
This problem is severe, but happens very infrequently, mostly on contenerized
environments. We have a lot of problems in tracker that we suspect are caused by this.
To find them, we have "problem-detection" PR #49198 in main.
Then we will apply equivalent solution there too.

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>