sys_quotacl tries to do a get_super on a struct block_device * to which
it doesn't hold a reference (nor does it actually have to be non-NULL).

As lookup bdev by name is a rather common operation I splitted out a new
helper, lookup_bdev() that does this out of open_bdev_excl and switched
quota.c to use it.  lookup_bdev() holds a proper reference that needs
to be dropped by bdput(), and it's well documented.