block: export blk_next_bio()

The block layer provides emulation of zone management operations
targeting all zones of a zoned block device only for the zone reset
operation (REQ_OP_ZONE_RESET). In order to correctly implement
exporting of zoned block devices with NVMeOF, emulating zone management
operations targeting all zones of a device is also necessary for the
open, close and finish zone operations (REQ_OP_ZONE_OPEN,
REQ_OP_ZONE_CLOSE and REQ_OP_ZONE_FINISH).

Instead of duplicating the code, export the existing helper from block
layer so we can use a bio chaining pattern that is present in the block
layer for REQ_OP_ZONE RESET all emulation in the NVMeOF zoned block
device backend.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>