f2fs: fix to do sanity check on sbi->total_valid_block_count

syzbot reported a f2fs bug as below:

------------[ cut here ]------------
kernel BUG at fs/f2fs/f2fs.h:2521!
RIP: 0010:dec_valid_block_count+0x3b2/0x3c0 fs/f2fs/f2fs.h:2521
Call Trace:
 f2fs_truncate_data_blocks_range+0xc8c/0x11a0 fs/f2fs/file.c:695
 truncate_dnode+0x417/0x740 fs/f2fs/node.c:973
 truncate_nodes+0x3ec/0xf50 fs/f2fs/node.c:1014
 f2fs_truncate_inode_blocks+0x8e3/0x1370 fs/f2fs/node.c:1197
 f2fs_do_truncate_blocks+0x840/0x12b0 fs/f2fs/file.c:810
 f2fs_truncate_blocks+0x10d/0x300 fs/f2fs/file.c:838
 f2fs_truncate+0x417/0x720 fs/f2fs/file.c:888
 f2fs_setattr+0xc4f/0x12f0 fs/f2fs/file.c:1112
 notify_change+0xbca/0xe90 fs/attr.c:552
 do_truncate+0x222/0x310 fs/open.c:65
 handle_truncate fs/namei.c:3466 [inline]
 do_open fs/namei.c:3849 [inline]
 path_openat+0x2e4f/0x35d0 fs/namei.c:4004
 do_filp_open+0x284/0x4e0 fs/namei.c:4031
 do_sys_openat2+0x12b/0x1d0 fs/open.c:1429
 do_sys_open fs/open.c:1444 [inline]
 __do_sys_creat fs/open.c:1522 [inline]
 __se_sys_creat fs/open.c:1516 [inline]
 __x64_sys_creat+0x124/0x170 fs/open.c:1516
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94

The reason is: in fuzzed image, sbi->total_valid_block_count is
inconsistent w/ mapped blocks indexed by inode, so, we should
not trigger panic for such case, instead, let's print log and
set fsck flag.

Fixes: 39a53e0 ("f2fs: add superblock and major in-memory structure")
Reported-by: syzbot+8b376a77b2f364097fbe@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/67f3c0b2.050a0220.396535.0547.GAE@google.com
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -2526,8 +2526,14 @@ static inline void dec_valid_block_count(struct f2fs_sb_info *sbi,
 	blkcnt_t sectors = count << F2FS_LOG_SECTORS_PER_BLOCK;
 
 	spin_lock(&sbi->stat_lock);
-	f2fs_bug_on(sbi, sbi->total_valid_block_count < (block_t) count);
-	sbi->total_valid_block_count -= (block_t)count;
+	if (unlikely(sbi->total_valid_block_count < count)) {
+		f2fs_warn(sbi, "Inconsistent total_valid_block_count:%u, ino:%lu, count:%u",
+			  sbi->total_valid_block_count, inode->i_ino, count);
+		sbi->total_valid_block_count = 0;
+		set_sbi_flag(sbi, SBI_NEED_FSCK);
+	} else {
+		sbi->total_valid_block_count -= count;
+	}
 	if (sbi->reserved_blocks &&
 		sbi->current_reserved_blocks < sbi->reserved_blocks)
 		sbi->current_reserved_blocks = min(sbi->reserved_blocks,