block: fold bdev_unhash_inode into invalidate_partition

invalidate_partition and bdev_unhash_inode are always paired, and
invalidate_partition already does an icache lookup for the block device
inode.  Piggy back on that to remove the inode from the hash.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>