fusefs: only test for incoherency if FN_SIZECHANGE is set
FUSE emits spurious incoherency warnings in writethrough mode. The
warnings are triggered by setattr calls generated by vnode truncation
turning the cached va_size vattr stale, causing comparisons with the
fresh version provided by the server to fail. Only validate the vnode's
va_size vattr if the FN_SIZECHANGE flag is set.
This is a part of the research work at RCSLab, University of Waterloo.