ext4: fix null-ptr-deref in ext4_write_info

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

ext4: fix null-ptr-deref in ext4_write_info

I caught a null-ptr-deref bug as follows:

==================================================================

KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]

CPU: 1 PID: 1589 Comm: umount Not tainted 5.10.0-02219-dirty #339

RIP: 0010:ext4_write_info+0x53/0x1b0

[...]

Call Trace:

dquot_writeback_dquots+0x341/0x9a0

ext4_sync_fs+0x19e/0x800

__sync_filesystem+0x83/0x100

sync_filesystem+0x89/0xf0

generic_shutdown_super+0x79/0x3e0

kill_block_super+0xa1/0x110

deactivate_locked_super+0xac/0x130

deactivate_super+0xb6/0xd0

cleanup_mnt+0x289/0x400

__cleanup_mnt+0x16/0x20

task_work_run+0x11c/0x1c0

exit_to_user_mode_prepare+0x203/0x210

syscall_exit_to_user_mode+0x5b/0x3a0

do_syscall_64+0x59/0x70

entry_SYSCALL_64_after_hwframe+0x44/0xa9

==================================================================

Above issue may happen as follows:

-------------------------------------

exit_to_user_mode_prepare

task_work_run

__cleanup_mnt

cleanup_mnt

deactivate_super

deactivate_locked_super

kill_block_super

generic_shutdown_super

shrink_dcache_for_umount

dentry = sb->s_root

sb->s_root = NULL <--- Here set NULL

sync_filesystem

__sync_filesystem

sb->s_op->sync_fs > ext4_sync_fs

dquot_writeback_dquots

sb->dq_op->write_info > ext4_write_info

ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2)

d_inode(sb->s_root)

s_root->d_inode <--- Null pointer dereference

To solve this problem, we use ext4_journal_start_sb directly

to avoid s_root being used.