mds: start a new major segment after reaching minor segment threshold

mds: start a new major segment after reaching minor segment threshold

Credit goes to Patrick (@batrick) for identifying this.

When there are huge number of subtree exports (such as done in export
thrashing test), the MDS would log an EExport event. The EExport event
is relatively large in size. This causes the MDS to log new minor log
segments frequently. Moreover, the MDS logs a major segment (boundary)
after a certain number of events have been logged. This casues large
number of (minor) events to get build up and cause delays in trimming
expired segments, since journal expire position is updated on segment
boundaries.

To mitigate this issue, the MDS now starts a major segment after a
configured number of minor segments have been logged. This threshold
is configurable by adjusting `mds_log_minor_segments_per_major_segment`
MDS config (defaults to 16).

Fixes: https://tracker.ceph.com/issues/66948
Signed-off-by: Venky Shankar <vshankar@redhat.com>