Small fix to the error-return path after calling

Small fix to the error-return path after calling
__copy_to_user in the swiotlb sync function. We cannot
immediately exit the sync function as there may be tail work
to do (e.g., kunmap_atomic). In fact, doing nothing is the
right thing to do and is sufficient to avoid compiler
warnings about not checking the __copy_to_user return value.

Signed-off-by: Keir Fraser <keir@xensource.com>