qcow2: Nicer variable names in qcow2_update_snapshot_refcount()

qcow2: Nicer variable names in qcow2_update_snapshot_refcount()

In order to keep checkpatch happy when the next patch changes
indentation, we first have to shorten some long lines.  The easiest
approach is to use a new variable in place of
'offset & L2E_OFFSET_MASK', except that 'offset' is the best name
for that variable.  Change '[old_]offset' to '[old_]entry' to
make room.

While touching things, also fix checkpatch warnings about unusual
'for' statements.

Suggested by Max Reitz <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Message-id: 20170507000552[email protected]
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Max Reitz <[email protected]>