Update copy_file_range(2) to be Linux5 compatible.

The current linux man page and testing done on a fairly recent linux5.n
kernel have identified two changes to the semantics of the linux
copy_file_range system call.
Since the copy_file_range(2) system call is intended to be linux compatible
and is only currently in head/current and not used by any commands,
it seems appropriate to update the system call to be compatible with
the current linux one.
The first of these semantic changes was changed to be compatible with
linux5.n by r354564.
For the second semantic change, the old linux man page stated that, if
infd and outfd referred to the same file, EBADF should be returned.
Now, the semantics is to allow infd and outfd to refer to the same file
so long as the byte ranges defined by the input file offset, output file offset
and len does not overlap. If the byte ranges do overlap, EINVAL should be
returned.
This patch modifies copy_file_range(2) to be linux5.n compatible for this
semantic change.