btrfs: do not increment dirty_metadata_bytes in set_btree_ioerr

We only add if we set the extent buffer dirty, and we subtract when we
clear the extent buffer dirty.  If we end up in set_btree_ioerr we have
already cleared the buffer dirty, and we aren't resetting dirty on the
extent buffer, so this is simply wrong.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>