f2fs: fix to don't dirty inode for readonly filesystem

syzbot reports f2fs bug as below:

kernel BUG at fs/f2fs/inode.c:933!
RIP: 0010:f2fs_evict_inode+0x1576/0x1590 fs/f2fs/inode.c:933
Call Trace:
 evict+0x2a4/0x620 fs/inode.c:664
 dispose_list fs/inode.c:697 [inline]
 evict_inodes+0x5f8/0x690 fs/inode.c:747
 generic_shutdown_super+0x9d/0x2c0 fs/super.c:675
 kill_block_super+0x44/0x90 fs/super.c:1667
 kill_f2fs_super+0x303/0x3b0 fs/f2fs/super.c:4894
 deactivate_locked_super+0xc1/0x130 fs/super.c:484
 cleanup_mnt+0x426/0x4c0 fs/namespace.c:1256
 task_work_run+0x24a/0x300 kernel/task_work.c:180
 ptrace_notify+0x2cd/0x380 kernel/signal.c:2399
 ptrace_report_syscall include/linux/ptrace.h:411 [inline]
 ptrace_report_syscall_exit include/linux/ptrace.h:473 [inline]
 syscall_exit_work kernel/entry/common.c:251 [inline]
 syscall_exit_to_user_mode_prepare kernel/entry/common.c:278 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:283 [inline]
 syscall_exit_to_user_mode+0x15c/0x280 kernel/entry/common.c:296
 do_syscall_64+0x50/0x110 arch/x86/entry/common.c:88
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

The root cause is:
- do_sys_open
 - f2fs_lookup
  - __f2fs_find_entry
   - f2fs_i_depth_write
    - f2fs_mark_inode_dirty_sync
     - f2fs_dirty_inode
      - set_inode_flag(inode, FI_DIRTY_INODE)

- umount
 - kill_f2fs_super
  - kill_block_super
   - generic_shutdown_super
    - sync_filesystem
    : sb is readonly, skip sync_filesystem()
    - evict_inodes
     - iput
      - f2fs_evict_inode
       - f2fs_bug_on(sbi, is_inode_flag_set(inode, FI_DIRTY_INODE))
       : trigger kernel panic

When we try to repair i_current_depth in readonly filesystem, let's
skip dirty inode to avoid panic in later f2fs_evict_inode().

Cc: stable@vger.kernel.org
Reported-by: syzbot+31e4659a3fe953aec2f4@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/000000000000e890bc0609a55cff@google.comSigned-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/inode.c

@@ -29,6 +29,9 @@ void f2fs_mark_inode_dirty_sync(struct inode *inode, bool sync)
 	if (is_inode_flag_set(inode, FI_NEW_INODE))
 		return;
 
+	if (f2fs_readonly(F2FS_I_SB(inode)->sb))
+		return;
+
 	if (f2fs_inode_dirtied(inode, sync))
 		return;