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 <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170507000552.20847-2-eblake@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
