logging: fs backend: simplify checking if file exists

In order to check if the desired log file exists, the backend would open
the directory, then go through all the files seeing if one of them matches
the correct filename. Simplify to just `fs_stat` the file instead.

This has the added side effect of lowering the time spent checking if the
file exists after every log. Some quick testing revealed the time spent
checking went down from ~150-300ms to ~10ms (on my specific board, with a
nRF9160 writing to a LittleFS on external flash).

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