block: simplify partition removal

Always look up the first available entry instead of the complicated
stateful traversal.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210406062303.811835-7-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>