btrfs: take into account global rsv in need_preemptive_reclaim

Global rsv can't be used for normal allocations, and for very full file
systems we can decide to try and async flush constantly even though
there's really not a lot of space to reclaim.  Deal with this by
including the global block rsv size in the "total used" calculation.
Reviewed-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -792,12 +792,14 @@ btrfs_calc_reclaim_metadata_size(struct btrfs_fs_info *fs_info,
 static bool need_preemptive_reclaim(struct btrfs_fs_info *fs_info,
 				    struct btrfs_space_info *space_info)
 {
+	u64 global_rsv_size = fs_info->global_block_rsv.reserved;
 	u64 ordered, delalloc;
 	u64 thresh = div_factor_fine(space_info->total_bytes, 98);
 	u64 used;
 
 	/* If we're just plain full then async reclaim just slows us down. */
-	if ((space_info->bytes_used + space_info->bytes_reserved) >= thresh)
+	if ((space_info->bytes_used + space_info->bytes_reserved +
+	     global_rsv_size) >= thresh)
 		return false;
 
 	/*