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>

drivers/md/dm.h

@@ -89,7 +89,6 @@ void dm_linear_exit(void);
 int dm_stripe_init(void);
 void dm_stripe_exit(void);
 
-void *dm_vcalloc(unsigned long nmemb, unsigned long elem_size);
 int dm_open_count(struct mapped_device *md);
 int dm_lock_for_deletion(struct mapped_device *md);