null_blk: use kmap_local_page() and kunmap_local()

Replace the deprecated API kmap_atomic() and kunmap_atomic() with
kmap_local_page() and kunmap_local() in null_flush_cache_page().

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230330184926.64209-2-kch@nvidia.com


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