f2fs: fix to avoid panic in dec_valid_node_count()

As Jungyeon reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=203213

- Overview
When mounting the attached crafted image and running program, I got this error.
Additionally, it hangs on sync after running the this script.

The image is intentionally fuzzed from a normal f2fs image for testing and I enabled option CONFIG_F2FS_CHECK_FS on.

- Reproduces
mkdir test
mount -t f2fs tmp.img test
cp a.out test
cd test
sudo ./a.out
sync

 kernel BUG at fs/f2fs/f2fs.h:2012!
 RIP: 0010:truncate_node+0x2c9/0x2e0
 Call Trace:
  f2fs_truncate_xattr_node+0xa1/0x130
  f2fs_remove_inode_page+0x82/0x2d0
  f2fs_evict_inode+0x2a3/0x3a0
  evict+0xba/0x180
  __dentry_kill+0xbe/0x160
  dentry_kill+0x46/0x180
  dput+0xbb/0x100
  do_renameat2+0x3c9/0x550
  __x64_sys_rename+0x17/0x20
  do_syscall_64+0x43/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The reason is dec_valid_node_count() will trigger kernel panic due to
inconsistent count in between inode.i_blocks and actual block.

To avoid panic, let's just print debug message and set SBI_NEED_FSCK to
give a hint to fsck for latter repairing.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix build warning and add unlikely]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -2029,7 +2029,6 @@ static inline void dec_valid_node_count(struct f2fs_sb_info *sbi,
 
 	f2fs_bug_on(sbi, !sbi->total_valid_block_count);
 	f2fs_bug_on(sbi, !sbi->total_valid_node_count);
-	f2fs_bug_on(sbi, !is_inode && !inode->i_blocks);
 
 	sbi->total_valid_node_count--;
 	sbi->total_valid_block_count--;
@@ -2039,10 +2038,19 @@ static inline void dec_valid_node_count(struct f2fs_sb_info *sbi,
 
 	spin_unlock(&sbi->stat_lock);
 
-	if (is_inode)
+	if (is_inode) {
 		dquot_free_inode(inode);
-	else
+	} else {
+		if (unlikely(inode->i_blocks == 0)) {
+			f2fs_msg(sbi->sb, KERN_WARNING,
+				"Inconsistent i_blocks, ino:%lu, iblocks:%llu",
+				inode->i_ino,
+				(unsigned long long)inode->i_blocks);
+			set_sbi_flag(sbi, SBI_NEED_FSCK);
+			return;
+		}
 		f2fs_i_blocks_write(inode, 1, false, true);
+	}
 }
 
 static inline unsigned int valid_node_count(struct f2fs_sb_info *sbi)