fusefs: don't track a file's size in two places

fuse_vnode_data.filesize was mostly redundant with
fuse_vnode_data.cached_attrs.st_size, but didn't have exactly the same
meaning. It was very confusing. This commit eliminates the former. It
also eliminates fuse_vnode_refreshsize, which ignored the cache timeout
value.

Sponsored by: The FreeBSD Foundation