Do not select zones with <5% free after upload (#9877)

Looking into full disk errors on zoned setup. We don't take the
5% space requirement into account when selecting a zone.

The interesting part is that even considering this we don't
know the size of the object the user wants to upload when
they do multipart uploads.

It seems quite defensive to always upload multiparts to
the zone where there is the most space since all load will
be directed to a part of the cluster.

In these cases we make sure it can at least hold a 1GiB file
and we disadvantage fuller zones more by subtracting the
expected size before weighing.

cmd/globals.go

@@ -96,6 +96,9 @@ const (
 
 	// Maximum size of default bucket encryption configuration allowed
 	maxBucketSSEConfigSize = 1 * humanize.MiByte
+
+	// diskFillFraction is the fraction of a disk we allow to be filled.
+	diskFillFraction = 0.95
 )
 
 var globalCLIContext = struct {