vfs: Put the definition of COPY_FILE_RANGE_CLONE in unistd.h

kib@ noted that having COPY_FILE_RANGE_CLONE defined
in vnode.h was inappropriate, since it is meant to be used from
user space as well as kernel space.

This patch moves it into sys/unistd.h and reverts the addition
of it in vnode.h.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D51838
Fixes: 37b2cb5ecb0f ("vfs: Add support for file cloning to VOP_COPY_FILE_RANGE")