fusefs: eliminate superfluous FUSE_GETATTR when filesize=0

fuse_vnode_refreshsize was using 0 as a flag value for filesize meaning
"uninitialized" (thanks to the malloc(...M_ZERO) in fuse_vnode_alloc. But
this led to unnecessary getattr operations when the filesize legitimately
happened to be zero. Fix by adding a distinct flag value.

Sponsored by: The FreeBSD Foundation