fix: fix removal of the upload directory in posix CompleteMultipartUpload

In the posix backend, the path argument to os.RemoveAll() does not
start with the bucket name. Since the path does not exist,
os.RemoveAll() does nothing and uploaded parts are left in the
".sgwtmp" directory.

This commit prefixes the path with the bucket name.

Fixes issue #626.

Signed-off-by: Christophe Vu-Brugier <christophe.vu-brugier@seagate.com>

backend/posix/posix.go

@@ -656,7 +656,7 @@ func (p *Posix) CompleteMultipartUpload(ctx context.Context, input *s3.CompleteM
 	}
 
 	// cleanup tmp dirs
-	os.RemoveAll(upiddir)
+	os.RemoveAll(filepath.Join(bucket, objdir, uploadID))
 	// use Remove for objdir in case there are still other uploads
 	// for same object name outstanding, this will fail if there are
 	os.Remove(filepath.Join(bucket, objdir))