f2fs: fix UAF issue in f2fs_merge_page_bio()

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

f2fs: fix UAF issue in f2fs_merge_page_bio()

As JY reported in bugzilla [1],

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
pc : [0xffffffe51d249484] f2fs_is_cp_guaranteed+0x70/0x98
lr : [0xffffffe51d24adbc] f2fs_merge_page_bio+0x520/0x6d4
CPU: 3 UID: 0 PID: 6790 Comm: kworker/u16:3 Tainted: P B W OE 6.12.30-android16-5-maybe-dirty-4k #1 5f7701c9cbf727d1eebe77c89bbbeb3371e895e5
Tainted: [P]=PROPRIETARY_MODULE, [B]=BAD_PAGE, [W]=WARN, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
Workqueue: writeback wb_workfn (flush-254:49)
Call trace:
f2fs_is_cp_guaranteed+0x70/0x98
f2fs_inplace_write_data+0x174/0x2f4
f2fs_do_write_data_page+0x214/0x81c
f2fs_write_single_data_page+0x28c/0x764
f2fs_write_data_pages+0x78c/0xce4
do_writepages+0xe8/0x2fc
__writeback_single_inode+0x4c/0x4b4
writeback_sb_inodes+0x314/0x540
__writeback_inodes_wb+0xa4/0xf4
wb_writeback+0x160/0x448
wb_workfn+0x2f0/0x5dc
process_scheduled_works+0x1c8/0x458
worker_thread+0x334/0x3f0
kthread+0x118/0x1ac
ret_from_fork+0x10/0x20

[1] https://bugzilla.kernel.org/show_bug.cgi?id=220575

The panic was caused by UAF issue w/ below race condition:

kworker
- writepages
- f2fs_write_cache_pages
- f2fs_write_single_data_page
- f2fs_do_write_data_page
- f2fs_inplace_write_data
- f2fs_merge_page_bio
- add_inu_page
: cache page #1 into bio & cache bio in
io->bio_list
- f2fs_write_single_data_page
- f2fs_do_write_data_page
- f2fs_inplace_write_data
- f2fs_merge_page_bio
- add_inu_page
: cache page #2 into bio which is linked
in io->bio_list
write
- f2fs_write_begin
: write page #1
- f2fs_folio_wait_writeback
- f2fs_submit_merged_ipu_write
- f2fs_submit_write_bio
: submit bio which inclues page #1 and #2

software IRQ
- f2fs_write_end_io
- fscrypt_free_bounce_page
: freed bounced page which belongs to page #2
- inc_page_count( , WB_DATA_TYPE(data_folio), false)
: data_folio points to fio->encrypted_page
the bounced page can be freed before
accessing it in f2fs_is_cp_guarantee()

It can reproduce w/ below testcase:
Run below script in shell #1:
for ((i=1;i>0;i++)) do xfs_io -f /mnt/f2fs/enc/file \
-c "pwrite 0 32k" -c "fdatasync"

Run below script in shell #2:
for ((i=1;i>0;i++)) do xfs_io -f /mnt/f2fs/enc/file \
-c "pwrite 0 32k" -c "fdatasync"

So, in f2fs_merge_page_bio(), let's avoid using fio->encrypted_page after
commit page into internal ipu cache.