f2fs: don't set RO when shutting down f2fs

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

f2fs: don't set RO when shutting down f2fs

Shutdown does not check the error of thaw_super due to readonly, which

causes a deadlock like below.

f2fs_ioc_shutdown(F2FS_GOING_DOWN_FULLSYNC) issue_discard_thread

- bdev_freeze

- freeze_super

- f2fs_stop_checkpoint()

- f2fs_handle_critical_error - sb_start_write

- set RO - waiting

- bdev_thaw

- thaw_super_locked

- return -EINVAL, if sb_rdonly()

- f2fs_stop_discard_thread

-> wait for kthread_stop(discard_thread);