bcache:remove a superfluous check in register_bcache

There have no reassign the bdev after check It is IS_ERR.
the double check !IS_ERR(bdev) is superfluous.

After commit 4e7b5671 ("block: remove i_bdev"),
"Switch the block device lookup interfaces to directly work with a dev_t
so that struct block_device references are only acquired by the
blkdev_get variants (and the blk-cgroup special case).  This means that
we now don't need an extra reference in the inode and can generally
simplify handling of struct block_device to keep the lookups contained
in the core block layer code."

so after lookup_bdev call, there no need to do bdput.

remove a superfluous check the bdev & don't call bdput after lookup_bdev.

Fixes: 4e7b5671

("block: remove i_bdev")
Signed-off-by: Yi Li <yili@winhong.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>