btrfs: qgroup: free per-trans reserved space when a subvolume gets dropped

[BUG]
Sometime fsstress could lead to qgroup warning for case like
generic/013:

  BTRFS warning (device dm-3): qgroup 0/259 has unreleased space, type 1 rsv 81920
  ------------[ cut here ]------------
  WARNING: CPU: 9 PID: 24535 at fs/btrfs/disk-io.c:4142 close_ctree+0x1dc/0x323 [btrfs]
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:close_ctree+0x1dc/0x323 [btrfs]
  Call Trace:
   btrfs_put_super+0x15/0x17 [btrfs]
   generic_shutdown_super+0x72/0x110
   kill_anon_super+0x18/0x30
   btrfs_kill_super+0x17/0x30 [btrfs]
   deactivate_locked_super+0x3b/0xa0
   deactivate_super+0x40/0x50
   cleanup_mnt+0x135/0x190
   __cleanup_mnt+0x12/0x20
   task_work_run+0x64/0xb0
   __prepare_exit_to_usermode+0x1bc/0x1c0
   __syscall_return_slowpath+0x47/0x230
   do_syscall_64+0x64/0xb0
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  ---[ end trace 6c341cdf9b6cc3c1 ]---
  BTRFS error (device dm-3): qgroup reserved space leaked

While that subvolume 259 is no longer in that filesystem.

[CAUSE]
Normally per-trans qgroup reserved space is freed when a transaction is
committed, in commit_fs_roots().

However for completely dropped subvolume, that subvolume is completely
gone, thus is no longer in the fs_roots_radix, and its per-trans
reserved qgroup will never be freed.

Since the subvolume is already gone, leaked per-trans space won't cause
any trouble for end users.

[FIX]
Just call btrfs_qgroup_free_meta_all_pertrans() before a subvolume is
completely dropped.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -5466,6 +5466,14 @@ int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref, int for_reloc)
 		}
 	}
 
+	/*
+	 * This subvolume is going to be completely dropped, and won't be
+	 * recorded as dirty roots, thus pertrans meta rsv will not be freed at
+	 * commit transaction time.  So free it here manually.
+	 */
+	btrfs_qgroup_convert_reserved_meta(root, INT_MAX);
+	btrfs_qgroup_free_meta_all_pertrans(root);
+
 	if (test_bit(BTRFS_ROOT_IN_RADIX, &root->state))
 		btrfs_add_dropped_root(trans, root);
 	else