libroot_build: Fix fs_read_attr() in fs_attr_untyped.cpp

* With a specified buffer size smaller than the attribute size the
  function would fail with ERANGE on Linux although it should just read
  as much as possible. Now we always read into our temporary data buffer
  with the full buffer size.
* Fix return value in case pos is > 0. pos must be subtracted from the
  bytes actually read.

src/build/libroot/fs_attr_untyped.cpp

@@ -523,8 +523,7 @@ fs_read_attr(int fd, const char *_attribute, uint32 type, off_t pos,
 
 	// read the attribute
 	char attributeBuffer[sizeof(AttributeHeader) + kMaxAttributeLength];
-	ssize_t bytesRead = min_c((size_t)kMaxAttributeLength, readBytes)
-		+ sizeof(AttributeHeader);
+	ssize_t bytesRead = sizeof(attributeBuffer);
 	if (localFD.Path()) {
 		bytesRead = get_attribute(-1, localFD.Path(), attribute.c_str(),
 			attributeBuffer, bytesRead);
@@ -549,12 +548,19 @@ fs_read_attr(int fd, const char *_attribute, uint32 type, off_t pos,
 	}
 
 	// copy the result into the provided buffer
-	bytesRead -= sizeof(AttributeHeader);
-	if (bytesRead > pos) {
+	bytesRead -= sizeof(AttributeHeader) + pos;
+	if (bytesRead < 0) {
+		// that means pos > <attribute size>
+		errno = B_BAD_VALUE;
+		return -1;
+	}
+
+	if (bytesRead > 0) {
+		if ((size_t)bytesRead > readBytes)
+			bytesRead = readBytes;
 		memcpy(buffer, attributeBuffer + sizeof(AttributeHeader) + pos,
-			bytesRead - pos);
-	} else
-		bytesRead = 0;
+			bytesRead);
+	}
 
 	return bytesRead;
 }