block: always define struct blk_integrity in genhd.h

This will reduce the amount of ifdefs inside the source code for various
drivers and also will reduce the amount of stub functions that were
created for the !CONFIG_BLK_DEV_INTEGRITY case.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>