btrfs: log at debug log when we ignore errors

This stuff is likely to fail in many setups (for example when quota is
not supported by the btrfs version), hence only log at debug
level. Previously we'd silently ignore things altogether which makes
things pretty hard to debug.

src/shared/machine-pool.c

@@ -378,14 +378,21 @@ int grow_machine_directory(void) {
                 new_size = old_size + max_add;
 
         r = btrfs_resize_loopback("/var/lib/machines", new_size, true);
-        if (r <= 0)
-                return r;
+        if (r < 0)
+                return log_debug_errno(r, "Failed to resize loopback: %m");
+        if (r == 0)
+                return 0;
 
         /* Also bump the quota, of both the subvolume leaf qgroup, as
          * well as of any subtree quota group by the same id but a
          * higher level, if it exists. */
-        (void) btrfs_qgroup_set_limit("/var/lib/machines", 0, new_size);
-        (void) btrfs_subvol_set_subtree_quota_limit("/var/lib/machines", 0, new_size);
+        r = btrfs_qgroup_set_limit("/var/lib/machines", 0, new_size);
+        if (r < 0)
+                log_debug_errno(r, "Failed to set btrfs limit: %m");
+
+        r = btrfs_subvol_set_subtree_quota_limit("/var/lib/machines", 0, new_size);
+        if (r < 0)
+                log_debug_errno(r, "Failed to set btrfs subtree limit: %m");
 
         log_info("Grew /var/lib/machines btrfs loopback file system to %s.", format_bytes(buf, sizeof(buf), new_size));
         return 1;