f2fs: support fs shutdown

This patch introduces a generic ioctl for fs shutdown, which was used by xfs.

If this shutdown is triggered, filesystem stops any further IOs according to the
following options.

1. FS_GOING_DOWN_FULLSYNC
 : this will flush all the data and dentry blocks, and do checkpoint before
   shutdown.

2. FS_GOING_DOWN_METASYNC
 : this will do checkpoint before shutdown.

3. FS_GOING_DOWN_NOSYNC
 : this will trigger shutdown as is.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>