Revert "Stop dropping caches now we have kernel fix"

This reverts commit 6abe6831b5.

Bringing this back temporarily for the same issue on sdcardfs.
Will remove once the kernel issue is resolved.

Change-Id: Ia29ea4fddb7777012a2eea9259f9ac856773fe01
Bug: 37231161
Test: Boot device with FBE enabled. ls /storage/emulated/0/Android
Unlock device. ls /storage/emulated/0/Android
1st will not be found. Second should be found.

Ext4Crypt.cpp

@@ -171,6 +171,13 @@ static bool install_key(const std::string& key, std::string* raw_ref) {
     LOG(DEBUG) << "Added key " << key_id << " (" << ref << ") to keyring " << device_keyring
                << " in process " << getpid();
 
+    // *TODO* Remove this code when kernel is fixed - see b/28373400
+    // Kernel preserves caches across a key insertion with ext4ice, which leads
+    // to contradictory dirents
+    if (!android::base::WriteStringToFile("3", "/proc/sys/vm/drop_caches")) {
+        PLOG(ERROR) << "Failed to drop_caches";
+    }
+
     return true;
 }