patch to add a Linux compatible copy_file_range(2) syscall

Hi,

I just put a patch in phabricator that is intended to add a Linux compatible
copy_file_range(2) syscall. My main interest in having this is that NFSv4.2 will
know how to do file copying locally on the NFS server, saving all the reads/writes
across the wire.

It copies the file byte range in the kernel. I don't know how the performance
compares with a userland file copy done to a local file system on the machine.
(It would save syscalls, but I have no idea if that will result in a noticeable
 performance difference?)

It is at https://reviews.freebsd.org/D20584
I've listed a few guys as possible reviewers, but if anyone else would like to review
it, feel free to add yourself.

If anyone is able to test this, it would be appreciated and let me know how it goes, rick.