shared: Ensure COPY_HOLES copies trailing holes

shared: Ensure COPY_HOLES copies trailing holes

Previously, files with a hole at the end would get silently truncated
which breaks reading journal files. This commit makes sure that holes
are punched in existing space and if no more space is available, that
we grow the file and the hole by using ftruncate().

The corresponding test is extended to put a hole at the end of the file
and we make sure that hole is copied correctly.