btrfs: kill min_allocable_bytes in inc_block_group_ro

	struct btrfs_space_info *sinfo = cache->space_info;

	u64 num_bytes;

	u64 sinfo_used;

	u64 min_allocable_bytes;

	int ret = -ENOSPC;

	/*

	 * We need some metadata space and system metadata space for

	 * allocating chunks in some corner cases until we force to set

	 * it to be readonly.

	 */

	if ((sinfo->flags &

	     (BTRFS_BLOCK_GROUP_SYSTEM | BTRFS_BLOCK_GROUP_METADATA)) &&

	    !force)

		min_allocable_bytes = SZ_1M;

	else

		min_allocable_bytes = 0;

	spin_lock(&sinfo->lock);

	spin_lock(&cache->lock);

	 * sinfo_used + num_bytes should always <= sinfo->total_bytes.

	 *

	 * Here we make sure if we mark this bg RO, we still have enough

	 * free space as buffer (if min_allocable_bytes is not 0).

	 * free space as buffer.

	 */

	if (sinfo_used + num_bytes + min_allocable_bytes <=

	    sinfo->total_bytes) {

	if (sinfo_used + num_bytes <= sinfo->total_bytes) {

		sinfo->bytes_readonly += num_bytes;

		cache->ro++;

		list_add_tail(&cache->ro_list, &sinfo->ro_bgs);

		btrfs_info(cache->fs_info,

			"unable to make block group %llu ro", cache->start);

		btrfs_info(cache->fs_info,

			"sinfo_used=%llu bg_num_bytes=%llu min_allocable=%llu",

			sinfo_used, num_bytes, min_allocable_bytes);

			"sinfo_used=%llu bg_num_bytes=%llu",

			sinfo_used, num_bytes);

		btrfs_dump_space_info(cache->fs_info, cache->space_info, 0, 0);

	}

	return ret;