blk_io_schedule() isn't called from performance sensitive code path, and
it is easier to maintain by exporting it as symbol.

Also blk_io_schedule() is only called by CONFIG_BLOCK code, so it is safe
to do this way. Meantime fixes build failure when CONFIG_BLOCK is off.

Cc: Christoph Hellwig <hch@infradead.org>
Fixes: e6249cdd ("block: add blk_io_schedule() for avoiding task hung in sync dio")
Reported-by: NSatya Tangirala <satyat@google.com>
Tested-by: NSatya Tangirala <satyat@google.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>