CVE-2021-46982 f2fs: compress: fix race condition of overwrite vs truncate

In the Linux kernel, the following vulnerability has been resolved:

f2fs: compress: fix race condition of overwrite vs truncate

pos_fsstress testcase complains a panic as belew:

------------[ cut here ]------------
kernel BUG at fs/f2fs/compress.c:1082!
invalid opcode: 0000 [#1] SMP PTI
CPU: 4 PID: 2753477 Comm: kworker/u16:2 Tainted: G OE 5.12.0-rc1-custom #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
Workqueue: writeback wb_workfn (flush-252:16)
RIP: 0010:prepare_compress_overwrite+0x4c0/0x760 [f2fs]
Call Trace:
f2fs_prepare_compress_overwrite+0x5f/0x80 [f2fs]
f2fs_write_cache_pages+0x468/0x8a0 [f2fs]
f2fs_write_data_pages+0x2a4/0x2f0 [f2fs]
do_writepages+0x38/0xc0
__writeback_single_inode+0x44/0x2a0
writeback_sb_inodes+0x223/0x4d0
__writeback_inodes_wb+0x56/0xf0
wb_writeback+0x1dd/0x290
wb_workfn+0x309/0x500
process_one_work+0x220/0x3c0
worker_thread+0x53/0x420
kthread+0x12f/0x150
ret_from_fork+0x22/0x30

The root cause is truncate() may race with overwrite as below,
so that one reference count left in page can not guarantee the
page attaching in mapping tree all the time, after truncation,
later find_lock_page() may return NULL pointer.

• prepare_compress_overwrite
• f2fs_pagecache_get_page
• unlock_page
  - f2fs_setattr
  - truncate_setsize
  - truncate_inode_page
  - delete_from_page_cache
• find_lock_page

Fix this by avoiding referencing updated page.