scsi: target: core: add missing file_{start,end}_write()

The callers of vfs_iter_write() are required to hold file_start_write().
file_start_write() is a no-op for the S_ISBLK() case, but it is really
needed when the backing file is a regular file.

We are going to move file_{start,end}_write() into vfs_iter_write(), but
we need to fix this first, so that the fix could be backported to stable
kernels.

Suggested-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/ZV8ETIpM+wZa33B5@infradead.org/
Cc:  <stable@vger.kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231123092000.2665902-1-amir73il@gmail.com
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>