copy: fix copying of extents beyond the apparent file size

copy: fix copying of extents beyond the apparent file size

fallocate can allocate extents beyond EOF via FALLOC_FL_KEEP_SIZE.
Where there is a gap (hole) between the extents, and EOF is within
that gap, the final hole wasn't reproduced, resulting in silent
data corruption in the copied file (size too small).

* src/copy.c (extent_copy): Ensure we don't process extents
beyond the apparent file size, since processing and allocating
those is not currently supported.
* tests/cp/fiemap-extents.sh: Renamed from tests/cp/fiemap-empty.sh
and re-enable parts checking the extents at and beyond EOF.
* tests/local.mk: Reference the renamed test.
* NEWS: Mention the bug fix.
Fixes http://bugs.gnu.org/21790