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.