dm persistent data: improve improve space map block alloc failure message

Improve space map error message when unable to allocate a new
metadata block.
Signed-off-by: NJoe Thornber <ejt@redhat.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/persistent-data/dm-space-map-metadata.c

@@ -337,7 +337,7 @@ static int sm_metadata_new_block(struct dm_space_map *sm, dm_block_t *b)
 {
 	int r = sm_metadata_new_block_(sm, b);
 	if (r)
-		DMERR("out of metadata space");
+		DMERR("unable to allocate new metadata block");
 	return r;
 }