usb-mtp: breakup MTP write into smaller chunks

usb-mtp: breakup MTP write into smaller chunks

For every MTP_WRITE_BUF_SZ copied, this patch writes it to file before
getting the next block of data. The file is kept opened for the
duration of the operation but the sanity checks on the write operation
are performed only once when the write operation starts. Additionally,
we also update the file size in the object metadata once the file has
completely been written.

Suggested-by: Gerd Hoffman <kraxel@redhat.com>
Signed-off-by: Bandan Das <bsd@redhat.com>
Message-id: 20190129131908.27924-3-bsd@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>