fiemap copy: add extent-scan.[ch], avoid a double-free and reorganize

Changes:
========
1. fix write_zeros() per Jim's comments.
2. remove char const *fname from struct extent_scan.
3. change the signature of open_extent_scan() from
"void open_extent_scan(struct extent_scan **scan)" to
"void open_extent_scan(struct extent_scan *scan)" to avoid having
to malloc the extent_scan variable; instead save it on the stack.
4. move close_extent_scan() from a function defined in extent-scan.c
to extent-scan.h as a macro definition, but it does nothing for now,
since initial extent scan defined at stack.
5. add a macro "free_extents_info()" defined at extent-scan.h to
release the memory allocated to extent info which should be called
combine with get_extents_info(), it just one line, so IMHO, define
it as macro should be ok.

* src/extent-scan.c: New file; functions to read "extents".
* src/extent-scan.h: Header file of extent-scan.c.
* src/Makefile.am: Reference it and link it to copy_source.
* src/copy.c: Use the new functions and avoid double-free.

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