f2fs: do checkpoint in kill_sb

When unmounting f2fs in force mode, we can get it stuck by io_schedule()
by some pending IOs in meta_inode.

io_schedule+0xd/0x30
wait_on_page_bit_common+0xc6/0x130
__filemap_fdatawait_range+0xbd/0x100
filemap_fdatawait_keep_errors+0x15/0x40
sync_inodes_sb+0x1cf/0x240
sync_filesystem+0x52/0x90
generic_shutdown_super+0x1d/0x110
kill_f2fs_super+0x28/0x80 [f2fs]
deactivate_locked_super+0x35/0x60
cleanup_mnt+0x36/0x70
task_work_run+0x79/0xa0
exit_to_usermode_loop+0x62/0x70
do_syscall_64+0xdb/0xf0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
0xffffffffffffffff

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

fs/f2fs/super.c

@@ -3089,9 +3089,19 @@ static struct dentry *f2fs_mount(struct file_system_type *fs_type, int flags,
 static void kill_f2fs_super(struct super_block *sb)
 {
 	if (sb->s_root) {
-		set_sbi_flag(F2FS_SB(sb), SBI_IS_CLOSE);
-		f2fs_stop_gc_thread(F2FS_SB(sb));
-		f2fs_stop_discard_thread(F2FS_SB(sb));
+		struct f2fs_sb_info *sbi = F2FS_SB(sb);
+
+		set_sbi_flag(sbi, SBI_IS_CLOSE);
+		f2fs_stop_gc_thread(sbi);
+		f2fs_stop_discard_thread(sbi);
+
+		if (is_sbi_flag_set(sbi, SBI_IS_DIRTY) ||
+				!is_set_ckpt_flags(sbi, CP_UMOUNT_FLAG)) {
+			struct cp_control cpc = {
+				.reason = CP_UMOUNT,
+			};
+			f2fs_write_checkpoint(sbi, &cpc);
+		}
 	}
 	kill_block_super(sb);
 }