vfs: Put the definition of COPY_FILE_RANGE_CLONE in an appropriate place

[rmacklem]

Summary

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.

[kib]

sys/sys/unistd.h

221

Why it matter which bits Linux uses?

[rmacklem]

sys/sys/unistd.h

221

It doesn't matter a lot. I thought it might avoid
confusion and simplify the Linuxolator implementation
(it wouldn't have to map flags).