Add permission bits to open with O_CREAT.

It's an error to pass open O_CREAT without giving it mode bits:
https://linux.die.net/man/3/open

Bug: 32073964
Test: Compiles with clang FORTIFY.
Change-Id: Iaa78c3003ed960e1ffe7e56623dba33db4e17443

storaged/tests/storaged_test.cpp

@@ -33,7 +33,7 @@
 
 static void pause(uint32_t sec) {
     const char* path = "/cache/test";
-    int fd = open(path, O_WRONLY | O_CREAT);
+    int fd = open(path, O_WRONLY | O_CREAT, 0600);
     ASSERT_LT(-1, fd);
     char buffer[2048];
     memset(buffer, 1, sizeof(buffer));