f2fs: fix potential panic during fstrim

As Ju Hyung Park reported:

"When 'fstrim' is called for manual trim, a BUG() can be triggered
randomly with this patch.

I'm seeing this issue on both x86 Desktop and arm64 Android phone.

On x86 Desktop, this was caused during Ubuntu boot-up. I have a
cronjob installed which calls 'fstrim -v /' during boot. On arm64
Android, this was caused during GC looping with 1ms gc_min_sleep_time
& gc_max_sleep_time."

Root cause of this issue is that f2fs_wait_discard_bios can only be
used by f2fs_put_super, because during put_super there must be no
other referrers, so it can ignore discard entry's reference count
when removing the entry, otherwise in other caller we will hit bug_on
in __remove_discard_cmd as there may be other issuer added reference
count in discard entry.

Thread A				Thread B
					- issue_discard_thread
- f2fs_ioc_fitrim
 - f2fs_trim_fs
  - f2fs_wait_discard_bios
   - __issue_discard_cmd
    - __submit_discard_cmd
					 - __wait_discard_cmd
					  - dc->ref++
					  - __wait_one_discard_bio
   - __wait_discard_cmd
    - __remove_discard_cmd
     - f2fs_bug_on(sbi, dc->ref)

Fixes: 969d1b180d987c2be02de890d0fff0f66a0e80de
Reported-by: Ju Hyung Park <qkrwngud825@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
static int f2fs_set_qf_name(struct super_block *sb, int qtype,
							substring_t *args)
{
	struct f2fs_sb_info *sbi = F2FS_SB(sb);
	char *qname;
	int ret = -EINVAL;

	if (sb_any_quota_loaded(sb) && !sbi->s_qf_names[qtype]) {
		f2fs_msg(sb, KERN_ERR,
			"Cannot change journaled "
			"quota options when quota turned on");
		return -EINVAL;
	}
	qname = match_strdup(args);
	if (!qname) {
		f2fs_msg(sb, KERN_ERR,
			"Not enough memory for storing quotafile name");
		return -EINVAL;
	}
	if (sbi->s_qf_names[qtype]) {
		if (strcmp(sbi->s_qf_names[qtype], qname) == 0)
			ret = 0;
		else
			f2fs_msg(sb, KERN_ERR,
				 "%s quota file already specified",
				 QTYPE2NAME(qtype));
		goto errout;
	}
	if (strchr(qname, '/')) {
		f2fs_msg(sb, KERN_ERR,
			"quotafile must be on filesystem root");
		goto errout;
	}
	sbi->s_qf_names[qtype] = qname;
	set_opt(sbi, QUOTA);
	return 0;
errout:
	kfree(qname);
	return ret;
}

static int f2fs_set_qf_name(struct super_block *sb, int qtype,
							substring_t *args)
{
	struct f2fs_sb_info *sbi = F2FS_SB(sb);
	char *qname;
	int ret = -EINVAL;

	if (sb_any_quota_loaded(sb) && !sbi->s_qf_names[qtype]) {
		f2fs_msg(sb, KERN_ERR,
			"Cannot change journaled "
			"quota options when quota turned on");
		return -EINVAL;
	}
	qname = match_strdup(args);
	if (!qname) {
		f2fs_msg(sb, KERN_ERR,
			"Not enough memory for storing quotafile name");
		return -EINVAL;
	}
	if (sbi->s_qf_names[qtype]) {
		if (strcmp(sbi->s_qf_names[qtype], qname) == 0)
			ret = 0;
		else
			f2fs_msg(sb, KERN_ERR,
				 "%s quota file already specified",
				 QTYPE2NAME(qtype));
		goto errout;
	}
	if (strchr(qname, '/')) {
		f2fs_msg(sb, KERN_ERR,
			"quotafile must be on filesystem root");
		goto errout;
	}
	sbi->s_qf_names[qtype] = qname;
	set_opt(sbi, QUOTA);
	return 0;
errout:
	kfree(qname);
	return ret;
}
