samples: fs: littlefs: fix lost flash area reference
samples: fs: littlefs: fix lost flash area reference

The API call to close the flash area was only invoked when the flash
area was erased.  It should be closed in all paths.

Signed-off-by: Peter A. Bigot <650538893330be855f76604594b4820eb226769e@pabigot.com>

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