dm thin metadata: remove duplicate pmd initialisation

Remove some duplicate initialisation of struct dm_pool_metadata.

These pmd fields are initialised by both:
  __format_metadata's calls to dm_btree_empty
  __write_initial_superblock + __begin_transaction

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-thin-metadata.c

@@ -512,11 +512,6 @@ static int __format_metadata(struct dm_pool_metadata *pmd)
 
 	__setup_btree_details(pmd);
 
-	pmd->root = 0;
-	pmd->details_root = 0;
-	pmd->trans_id = 0;
-	pmd->flags = 0;
-
 	r = dm_btree_empty(&pmd->info, &pmd->root);
 	if (r < 0)
 		goto bad_data_sm;