block: continue until base is found in bdrv_freeze_backing_chain() et al

block: continue until base is found in bdrv_freeze_backing_chain() et al

All three functions that handle the BdrvChild.frozen attribute walk
the backing chain from 'bs' to 'base' and stop either when 'base' is
found or at the end of the chain if 'base' is NULL.

However if 'base' is not found then the functions return without
errors as if it was NULL.

This is wrong: if the caller passed an incorrect parameter that means
that there is a bug in the code.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>