dm: export dm get md

Export dm_get_md() for the new thin provisioning target to use.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 9836324..6b6616a 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2250,6 +2250,7 @@
 
 	return md;
 }
+EXPORT_SYMBOL_GPL(dm_get_md);
 
 void *dm_get_mdptr(struct mapped_device *md)
 {