f2fs: trigger correct checkpoint during umount

This patch fixes to trigger checkpoint with umount flag when kill_sb was called.
In kill_sb, f2fs_sync_fs was finally called, but at this time, f2fs can't do
checkpoint with CP_UMOUNT.
After then, f2fs_put_super is not doing checkpoint, since it is not dirty.

So, this patch adds a flag to indicate f2fs_sync_fs is called during umount.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -525,6 +525,7 @@ struct f2fs_sb_info {
 	struct f2fs_super_block *raw_super;	/* raw super block pointer */
 	int s_dirty;				/* dirty flag for checkpoint */
 	bool need_fsck;				/* need fsck.f2fs to fix */
+	bool s_closing;				/* specify unmounting */
 
 	/* for node-related operations */
 	struct f2fs_nm_info *nm_info;		/* node manager */