nfsclient copy_file_range(): flush dst vnode data
Otherwise server-side copy makes the client cache inconsistent with the
server data.
Reported by: asomers
PR: 276002
Reviewed by: rmacklem
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D43250