fusefs: writes should update the file size, even when data_cache_mode=0

fusefs: writes should update the file size, even when data_cache_mode=0

Writes that extend a file should update the file's size.  r344185 restricted
that behavior for fusefs to only happen when the data cache was enabled.
That probably made sense at the time because the attribute cache wasn't
fully baked yet.  Now that it is, we should always update the cached file
size during write.

Sponsored by:	The FreeBSD Foundation