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>