f2fs: fix to avoid panic in dec_valid_block_count()

As Jungyeon reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=203209

- Overview
When mounting the attached crafted image and running program, I got this error.
Additionally, it hangs on sync after 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
cc poc_01.c
./run.sh f2fs
sync

 kernel BUG at fs/f2fs/f2fs.h:1788!
 RIP: 0010:f2fs_truncate_data_blocks_range+0x342/0x350
 Call Trace:
  f2fs_truncate_blocks+0x36d/0x3c0
  f2fs_truncate+0x88/0x110
  f2fs_setattr+0x3e1/0x460
  notify_change+0x2da/0x400
  do_truncate+0x6d/0xb0
  do_sys_ftruncate+0xf1/0x160
  do_syscall_64+0x43/0xf0
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The reason is dec_valid_block_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

@@ -1789,6 +1789,7 @@ static inline int inc_valid_block_count(struct f2fs_sb_info *sbi,
 	return -ENOSPC;
 }
 
+void f2fs_msg(struct super_block *sb, const char *level, const char *fmt, ...);
 static inline void dec_valid_block_count(struct f2fs_sb_info *sbi,
 						struct inode *inode,
 						block_t count)
@@ -1797,13 +1798,21 @@ static inline void dec_valid_block_count(struct f2fs_sb_info *sbi,
 
 	spin_lock(&sbi->stat_lock);
 	f2fs_bug_on(sbi, sbi->total_valid_block_count < (block_t) count);
-	f2fs_bug_on(sbi, inode->i_blocks < sectors);
 	sbi->total_valid_block_count -= (block_t)count;
 	if (sbi->reserved_blocks &&
 		sbi->current_reserved_blocks < sbi->reserved_blocks)
 		sbi->current_reserved_blocks = min(sbi->reserved_blocks,
 					sbi->current_reserved_blocks + count);
 	spin_unlock(&sbi->stat_lock);
+	if (unlikely(inode->i_blocks < sectors)) {
+		f2fs_msg(sbi->sb, KERN_WARNING,
+			"Inconsistent i_blocks, ino:%lu, iblocks:%llu, sectors:%llu",
+			inode->i_ino,
+			(unsigned long long)inode->i_blocks,
+			(unsigned long long)sectors);
+		set_sbi_flag(sbi, SBI_NEED_FSCK);
+		return;
+	}
 	f2fs_i_blocks_write(inode, count, false, true);
 }
 
@@ -2819,7 +2828,6 @@ static inline void f2fs_update_iostat(struct f2fs_sb_info *sbi,
 
 bool f2fs_is_valid_blkaddr(struct f2fs_sb_info *sbi,
 					block_t blkaddr, int type);
-void f2fs_msg(struct super_block *sb, const char *level, const char *fmt, ...);
 static inline void verify_blkaddr(struct f2fs_sb_info *sbi,
 					block_t blkaddr, int type)
 {