dm: publish dm_vcalloc

Publish dm_vcalloc in include/linux/device-mapper.h because this function is
used by targets.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index e462c7f..08d7835 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -267,6 +267,11 @@
  */
 int dm_swap_table(struct mapped_device *md, struct dm_table *t);
 
+/*
+ * A wrapper around vmalloc.
+ */
+void *dm_vcalloc(unsigned long nmemb, unsigned long elem_size);
+
 /*-----------------------------------------------------------------
  * Macros.
  *---------------------------------------------------------------*/