fs: add kerneldoc to file_{start,end}_write() helpers

and use sb_end_write() instead of open coded version.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Message-Id: <20230817141337.1025891-3-amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>