btrfs: update superblock's device bytes_used when dropping chunk

Each superblock contains a copy of the device item for that device. In a
transaction which drops a chunk but doesn't create any new ones, we were
correctly updating the device item in the chunk tree but not copying
over the new bytes_used value to the superblock.

This can be seen by doing the following:

 # cd
 # dd if=/dev/zero of=test bs=4096 count=2621440
 # mkfs.btrfs test
 # mount test /root/temp

 # cd /root/temp
 # for i in {00..10}; do dd if=/dev/zero of=$i bs=4096 count=32768; done
 # sync
 # rm *
 # sync
 # btrfs balance start -dusage=0 .
 # sync

 # cd
 # umount /root/temp
 # btrfs check test

(For btrfs-check to detect this, you will also need my patch at
kdave/btrfs-progs#991.)

Change btrfs_remove_dev_extents() so that it adds the devices to the
post_commit_list if they're not there already. This causes
btrfs_commit_device_sizes() to be called, which updates the bytes_used
value in the superblock.

Signed-off-by: Mark Harmstone <maharmstone@fb.com>

Author: maharmstone

fs/btrfs/volumes.c

@@ -3282,6 +3282,12 @@ int btrfs_remove_chunk(struct btrfs_trans_handle *trans, u64 chunk_offset)
 					device->bytes_used - dev_extent_len);
 			atomic64_add(dev_extent_len, &fs_info->free_chunk_space);
 			btrfs_clear_space_info_full(fs_info);
+
+			if (list_empty(&device->post_commit_list)) {
+				list_add_tail(&device->post_commit_list,
+					      &trans->transaction->dev_update_list);
+			}
+
 			mutex_unlock(&fs_info->chunk_mutex);
 		}
 	}