btrfs: fix calculation of the global block reserve's size

At btrfs_update_global_block_rsv(), we are assuming an unlink operation
uses 5 metadata units, but that's not true anymore, it uses 6 since the
commit bca4ad7c ("btrfs: reserve correct number of items for unlink
and rmdir"). So update the code and comments to consider 6 units.

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