Commit df9a2850 introduced force_fsync to resolve the placement contract
conflict (EAGAIN) when a new metadata entry depends on a previous unsynced
entry that is already in-flight. However, it mistakenly grouped the
'force_fsync > 0' condition together with the Superblock Checkpoint logic.
As a result, every time the internal torn-write prevention mechanism
requested a force_fsync, the system inadvertently executed an expensive
trim_lsn() (updating the Superblock with RWF_DSYNC), causing unnecessary
write latency and I/O amplification during congested object updates.
This change minimally refactors the state machine to ensure trim_lsn() is
strictly bound to the intended journal_trim_interval, while force_fsync
only triggers the necessary fsync_buffer().
