btrfs: change the minimum global reserve size

	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;

	struct btrfs_space_info *sinfo = block_rsv->space_info;

	u64 num_bytes;

	unsigned min_items;

	/*

	 * The global block rsv is based on the size of the extent tree, the

	num_bytes = btrfs_root_used(&fs_info->extent_root->root_item) +

		btrfs_root_used(&fs_info->csum_root->root_item) +

		btrfs_root_used(&fs_info->tree_root->root_item);

	num_bytes = max_t(u64, num_bytes, SZ_16M);

	/*

	 * We at a minimum are going to modify the csum root, the tree root, and

	 * the extent root.

	 */

	min_items = 3;

	/*

	 * But we also want to reserve enough space so we can do the fallback

	 * global reserve for an unlink, which is an additional 5 items (see the

	 * comment in __unlink_start_trans for what we're modifying.)

	 *

	 * But we also need space for the delayed ref updates from the unlink,

	 * so its 10, 5 for the actual operation, and 5 for the delayed ref

	 * updates.

	 */

	min_items += 10;

	num_bytes = max_t(u64, num_bytes,

			  btrfs_calc_insert_metadata_size(fs_info, min_items));

	spin_lock(&sinfo->lock);

	spin_lock(&block_rsv->lock);