f2fs: support f2fs by setting unmovable bit for package file

This enables to use uncrypt for f2fs update-on-reboot.
It requires kernel patch named:
    "f2fs: add an ioctl to disable GC for specific file"

If any operation fails during uncrypt, please delete package file as soon as
possible, and create the file again to move forward. IOWs, don't leave the
package file for a long time.

Bug: 70309376
Bug: 30170612
Change-Id: I3b4233e7da756f107be35364521699deaf2e7139
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>