smp: export __smp_call_function_single()

The blk-mq core and the blk-mq null driver uses it.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/kernel/smp.c b/kernel/smp.c
index 0564571..46d3d0b 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -340,6 +340,7 @@
 	}
 	put_cpu();
 }
+EXPORT_SYMBOL_GPL(__smp_call_function_single);
 
 /**
  * smp_call_function_many(): Run a function on a set of other CPUs.