Throw 404 for head requests for prefixes without trailing "/" (#5966)

Minio server returns 403 (access denied) for head requests to prefixes
without trailing "/", this is different from S3 behaviour. S3 returns
404 in such cases.

Fixes #6080

cmd/fs-v1-helpers_test.go

@@ -136,7 +136,7 @@ func TestFSStats(t *testing.T) {
 			srcFSPath:   path,
 			srcVol:      "success-vol",
 			srcPath:     "path",
-			expectedErr: errFileAccessDenied,
+			expectedErr: errFileNotFound,
 		},
 		// Test case - 6.
 		// Test case with src path segment > 255.