Drivers: hv: fcopy: restore correct transfer length

Till recently the expected length of bytes read by the
daemon did depend on the context. It was either hv_start_fcopy or
hv_do_fcopy. The daemon had a buffer size of two pages, which was much
larger than needed.

Now the expected length of bytes read by the
daemon changed slightly. For START_FILE_COPY it is still the size of
hv_start_fcopy.  But for WRITE_TO_FILE and the other operations it is as
large as the buffer that arrived via vmbus. In case of WRITE_TO_FILE
that is slightly larger than a struct hv_do_fcopy. Since the buffer in
the daemon was still larger everything was fine.

Currently, the daemon reads only what is actually needed.
The new buffer layout is as large as a struct hv_do_fcopy, for the
WRITE_TO_FILE operation. Since the kernel expects a slightly larger
size, hvt_op_read will return -EINVAL because the daemon will read
slightly less than expected. Address this by restoring the expected
buffer size in case of WRITE_TO_FILE.

Fixes: 'c7e490fc ("Drivers: hv: fcopy: convert to hv_utils_transport")'
Fixes: '3f2baa8a ("Tools: hv: update buffer handling in hv_fcopy_daemon")'

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>