copy: disallow copy_file_range() on Linux kernels before 5.3

copy: disallow copy_file_range() on Linux kernels before 5.3

copy_file_range() before Linux kernel release 5.3 had many issues,
as described at https://lwn.net/Articles/789527/, which was
referenced from https://lwn.net/Articles/846403/; a more general
article discussing the generality of copy_file_range().
Linux kernel 5.3 was released in September 2019, which is new enough
that we need to actively avoid older kernels.

* src/copy.c (functional_copy_file_range): A new function
that returns false for Linux kernels before version 5.3.
(sparse_copy): Call this new function to gate use of
copy_file_range().