basic/copy: use copy_file_range()

basic/copy: use copy_file_range()

For btrfs, c_f_r() is like BTRFS_IOC_CLONE which we already used, but also
works when max_bytes is set. We do call copy_bytes in coredump code with
max_bytes set, and for large files, so we might see some benefit from using
c_f_r() on btrfs.

For other filesystems, c_f_r() falls back to do_splice_direct(), the same as
sendfile, which we already call, so there shouldn't be much difference.

Tested with test-copy and systemd-coredump on Linux 4.3 (w/o c_f_r)
and 4.5 (w/ c_f_r).