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

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

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().