os/fs/smartfs : Add buffering of data to be written to smartfs file
os/fs/smartfs : Add buffering of data to be written to smartfs file

- Hold the buffer from being committed to flash if the buffer is not completely full.
- The buffer will get synced when the file is closed or if another write request is
  issued which fills up the left over space in the buffer.

Signed-off-by: Taejun Kwon <6b00afae7efa03b5711c14fda48a12632e051a8c@samsung.com>, Amogh Hassija <a.hassija@samsung.com>

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