uncrypt: write permission for f2fs_pin_file

We need a write permission to set a flag in the file.

Change-Id: I4896ecbe0fc04374e01d006b1c8acdb932e5d16d
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>

uncrypt/uncrypt.cpp

@@ -300,7 +300,7 @@ static int produce_block_map(const char* path, const char* map_file, const char*
     int head_block = 0;
     int head = 0, tail = 0;
 
-    android::base::unique_fd fd(open(path, O_RDONLY));
+    android::base::unique_fd fd(open(path, O_RDWR));
     if (fd == -1) {
         PLOG(ERROR) << "failed to open " << path << " for reading";
         return kUncryptFileOpenError;