btrfs: fix reclaimed bytes accounting after automatic block group reclaim

btrfs: fix reclaimed bytes accounting after automatic block group reclaim

We are considering the used bytes counter of a block group as the amount
to update the space info's reclaim bytes counter after relocating the
block group, but this value alone is often not enough. This is because we
may have a reserved extent (or more) and in that case its size is
reflected in the reserved counter of the block group - the size of the
extent is only transferred from the reserved counter to the used counter
of the block group when the delayed ref for the extent is run - typically
when committing the transaction (or when flushing delayed refs due to
ENOSPC on space reservation). Such call chain for data extents is:

   btrfs_run_delayed_refs_for_head()
       run_one_delayed_ref()
           run_delayed_data_ref()
               alloc_reserved_file_extent()
                   alloc_reserved_extent()
                       btrfs_update_block_group()
                          -> transfers the extent size from the reserved
                             counter to the used counter

For metadata extents:

   btrfs_run_delayed_refs_for_head()
       run_one_delayed_ref()
           run_delayed_tree_ref()
               alloc_reserved_tree_block()
                   alloc_reserved_extent()
                       btrfs_update_block_group()
                           -> transfers the extent size from the reserved
                              counter to the used counter

Since relocation flushes delalloc, waits for ordered extent completion
and commits the current transaction before doing the actual relocation
work, the correct amount of reclaimed space is therefore the sum of the
"used" and "reserved" counters of the block group before we call
btrfs_relocate_chunk() at btrfs_reclaim_bgs_work().

So fix this by taking the "reserved" counter into consideration.

Fixes: 243192b67649 ("btrfs: report reclaim stats in sysfs")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>