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:	freebsd#1110

Author: etsal

sys/fs/fuse/fuse_internal.c

@@ -269,10 +269,10 @@ fuse_internal_cache_attrs(struct vnode *vp, struct fuse_attr *attr,
 
 	if (vnode_isreg(vp) &&
 	    fvdat->cached_attrs.va_size != VNOVAL &&
+	    fvdat->flag & FN_SIZECHANGE &&
 	    attr->size != fvdat->cached_attrs.va_size)
 	{
-		if ( data->cache_mode == FUSE_CACHE_WB &&
-		    fvdat->flag & FN_SIZECHANGE)
+		if (data->cache_mode == FUSE_CACHE_WB)
 		{
 			const char *msg;