logging: fs backend: only fs_sync when done with a batch of logs

The previous behavior was to call `fs_sync` on the file after every log
item written. In many cases, log messages come in bursts, and `log_core`
already operates in batches, configurable using
`CONFIG_LOG_PROCESS_THREAD_SLEEP_MS` and
`CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD`. Once the batch is done, the
backends get notified with `LOG_BACKEND_EVT_PROCESS_THREAD_DONE` if any
logs were processed. This is a nice time to `fs_sync` the written changes.

This change both speeds up the time spent writing logs and reduces the
amount of block erases done on the flash, as the previous implementation
erased blocks after each individual log message.

Signed-off-by: Kamil Krzyzanowski <kamnxt@kamnxt.com>