dm-writecache: use bvec_kmap_local instead of bvec_kmap_irq

dm-writecache: use bvec_kmap_local instead of bvec_kmap_irq

There is no need to disable interrupts in bio_copy_block, and the local
only mappings helps to avoid any sort of problems with stray writes
into the bio data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-8-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>