btrfs: stop reserving excessive space for block group item updates

Space for block group item updates, necessary after allocating or
deallocating an extent from a block group, is reserved in the delayed
refs block reserve. Currently we do this by incrementing the transaction
handle's delayed_ref_updates counter and then calling
btrfs_update_delayed_refs_rsv(), which will increase the size of the
delayed refs block reserve by an amount that corresponds to the same
amount we use for delayed refs, given by btrfs_calc_delayed_ref_bytes().

That is an excessive amount because it corresponds to the amount of space
needed to insert one item in a btree (btrfs_calc_insert_metadata_size())
times 2 when the free space tree feature is enabled. All we need is an
amount as given by btrfs_calc_metadata_size(), since we only need to
update an existing block group item in the extent tree (or block group
tree if this feature is enabled). By using btrfs_calc_metadata_size() we
will need to reserve 4 times less space when using the free space tree
and 2 times less space when not using it, putting less pressure on space
reservation.

So use helpers to reserve and release space for block group item updates
that use btrfs_calc_metadata_size() for calculation of the space.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>