uncrypt: write permission for f2fs_pin_file

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

Bug: None
Test: pre-submit

Change-Id: I4896ecbe0fc04374e01d006b1c8acdb932e5d16d
Merged-In: I4896ecbe0fc04374e01d006b1c8acdb932e5d16d
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
(cherry picked from commit 051b9d87)