vfs: copy_file_range() between multiple mountpoints of the same fs type

VOP_COPY_FILE_RANGE(9) is now caled when source and target vnodes
reside on the same filesystem type (not just on the same mountpoint).
The check if vnodes are on the same mountpoint must be done in the
filesystem code. There are currently only three users - fusefs(5) already
has this check, ZFS can handle multiple mountpoints and a check has been
added to NFS client.

ZFS block cloning is now possible between all snapshots and datasets
of the same ZFS pool.

This is an early MFC due to release schedule.

Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D41721
Approved by: re (gjb)

(cherry picked from commit 969071be938ca9b96f8dff003c7b43d6308849f1)