f2fs: fix to drop meta/node pages during umount

f2fs: fix to drop meta/node pages during umount

As reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=204193

A null pointer dereference bug is triggered in f2fs under kernel-5.1.3.

 kasan_report.cold+0x5/0x32
 f2fs_write_end_io+0x215/0x650
 bio_endio+0x26e/0x320
 blk_update_request+0x209/0x5d0
 blk_mq_end_request+0x2e/0x230
 lo_complete_rq+0x12c/0x190
 blk_done_softirq+0x14a/0x1a0
 __do_softirq+0x119/0x3e5
 irq_exit+0x94/0xe0
 call_function_single_interrupt+0xf/0x20

During umount, we will access NULL sbi->node_inode pointer in
f2fs_write_end_io():

	f2fs_bug_on(sbi, page->mapping == NODE_MAPPING(sbi) &&
				page->index != nid_of_node(page));

The reason is if disable_checkpoint mount option is on, meta dirty
pages can remain during umount, and then be flushed by iput() of
meta_inode, however node_inode has been iput()ed before
meta_inode's iput().

Since checkpoint is disabled, all meta/node datas are useless and
should be dropped in next mount, so in umount, let's adjust
drop_inode() to give a hint to iput_final() to drop all those dirty
datas correctly.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>