block: move blk_drop_partitions to blk.h

No need to have this declaration in a public header.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20220124093913.742411-3-hch@lst.de


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