block: Fix off-by-one errors in blk_status_to_errno() and print_req_error()

This was detected by the smatch static analyzer.

Fixes: commit 2a842aca

 ("block: introduce new block status code type")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>