fd_file: never report Close failure on linux.

close() will always succeed on linux, even if it returns -1 and sets
errno. Always report success, to avoid potential double closes.

Test: fd_file_test on host
Change-Id: I7d47b978645beff6b769f2ae05c32b0e328e433d
diff --git a/runtime/base/unix_file/fd_file.cc b/runtime/base/unix_file/fd_file.cc
index 58b047f..792c581 100644
--- a/runtime/base/unix_file/fd_file.cc
+++ b/runtime/base/unix_file/fd_file.cc
@@ -163,13 +163,18 @@
     moveUp(GuardState::kClosed, nullptr);
   }
 
+#if defined(__linux__)
+  // close always succeeds on linux, even if failure is reported.
+  UNUSED(result);
+#else
   if (result == -1) {
     return -errno;
-  } else {
-    fd_ = -1;
-    file_path_ = "";
-    return 0;
   }
+#endif
+
+  fd_ = -1;
+  file_path_ = "";
+  return 0;
 }
 
 int FdFile::Flush() {