NTB: ntb_tool: uninitialized heap data in tool_fn_write()

The call to:

	ret = simple_write_to_buffer(buf, size, offp, ubuf, size);

will return success if it is able to write even one byte to "buf".
The value of "*offp" controls which byte.  This could result in
reading uninitialized data when we do the sscanf() on the next line.

This code is not really desigined to handle partial writes where
*offp is non-zero and the "buf" is preserved and re-used between writes.
Just ban partial writes and replace the simple_write_to_buffer() with
copy_from_user().

Fixes: 578b881b ("NTB: Add tool test client")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>