Avoid unnecessary map allocation when writing progress

`MultiWriter` would create an unnecessary map allocation when the
`Write` method was used. The `Write` method would create a progress
object with the meta field initialized to the meta field of the writer
itself. It then invoked its own `WriteRawProgress` method which would
see two maps with the metadata and erroneously believe that they were
different and needed to be merged into a single metadata map.

Since this map is initialized with the metadata of the writer before
`WriteRawProgress` is invoked, this merge was unnecessary and could add
a lot of unnecessary memory allocations during a build.

This changes the `MultiWriter.Write` method to invoke the private
`writeRawProgress` which performs the actual write and avoids the
metadata merge.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
(cherry picked from commit 4c83fcf)

Author: jsternberg

util/progress/multiwriter.go

@@ -65,7 +65,7 @@ func (ps *MultiWriter) Write(id string, v interface{}) error {
 		Sys:       v,
 		meta:      ps.meta,
 	}
-	return ps.WriteRawProgress(p)
+	return ps.writeRawProgress(p)
 }
 
 func (ps *MultiWriter) WriteRawProgress(p *Progress) error {