copy: make extent_copy use sparse_copy, rather than its own code

copy: make extent_copy use sparse_copy, rather than its own code

* src/copy.c (extent_copy): Before this change, extent_copy would fail
to create holes, thus breaking --sparse=auto and --sparse=always.
I.e., copying a large enough file of all zeros, cp --sparse=always
should introduce a hole, but with extent_copy, it would not.