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.

Reviewed by: asomers
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1110