copy: do not treat unwritten extents specially: avoid XFS/ext4 data loss

copy: do not treat unwritten extents specially: avoid XFS/ext4 data loss

* src/copy.c (extent_copy): Do not treat "unwritten extents" specially.
Otherwise, with a release-candidate 2.6.39-rc3 kernel, XFS or ext4,
when using gold as your linker, and if you forget to run "make check",
you could end up installing files full of zeros instead of the expected
binaries.  For a lot of discussion, see
http://thread.gmane.org/gmane.comp.file-systems.xfs.general/37895
* tests/cp/fiemap-empty: Disable this test.