chunk store: handle insertion edge cases

chunk store: handle insertion edge cases

these were previously called out in a comment, but should now be handled (as
much as they can be).

the performance impact shouldn't be too bad, since we only look at the magic 8
bytes at the start of the existing chunk (we already did a stat on it, so that
might even be prefetched already by storage), and only if there is a size
mismatch and encryption is enabled.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
 [ T: fold in "just to be sure" touch_chunk calls ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>