dm thin metadata: call precommit before saving the roots

These calls were the wrong way round in __write_initial_superblock.

Cc: [e-mail]
Signed-off-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-thin-metadata.c

···
 	if (r < 0)
 		return r;
 
-	r = save_sm_roots(pmd);
+	r = dm_tm_pre_commit(pmd->tm);
 	if (r < 0)
 		return r;
 
-	r = dm_tm_pre_commit(pmd->tm);
+	r = save_sm_roots(pmd);
 	if (r < 0)
 		return r;