vfio-ccw: Do not read region ret_code after write

vfio-ccw: Do not read region ret_code after write

A pwrite() call returns the number of bytes written (or -1 on error),
and vfio-ccw compares this number with the size of the region to
determine if an error had occurred or not.

If they are not equal, this is a failure and the errno is used to
determine exactly how things failed. An errno of zero is possible
(though unlikely) in this situation and would be translated to a
successful operation.

If they ARE equal, the ret_code field is read from the region to
determine how to proceed. While the kernel sets the ret_code field
as necessary, the region and thus this field is not "written back"
to the user. So the value can only be what it was initialized to,
which is zero.

So, let's convert an unexpected length with errno of zero to a
return code of -EFAULT, and explicitly set an expected length to
a return code of zero. This will be a little safer and clearer.

Suggested-by: Matthew Rosato <[email protected]>
Signed-off-by: Eric Farman <[email protected]>
Message-Id: <20210303160739.2179378[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>