get rid of s_files and files_lock

The only thing we need it for is alt-sysrq-r (emergency remount r/o)
and these days we can do just as well without going through the
list of files.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
bool grab_super_passive(struct super_block *sb)
{
	spin_lock(&sb_lock);
	if (hlist_unhashed(&sb->s_instances)) {
		spin_unlock(&sb_lock);
		return false;
	}

	sb->s_count++;
	spin_unlock(&sb_lock);

	if (down_read_trylock(&sb->s_umount)) {
		if (sb->s_root && (sb->s_flags & MS_BORN))
			return true;
		up_read(&sb->s_umount);
	}

	put_super(sb);
	return false;
}

bool grab_super_passive(struct super_block *sb)
{
	spin_lock(&sb_lock);
	if (hlist_unhashed(&sb->s_instances)) {
		spin_unlock(&sb_lock);
		return false;
	}

	sb->s_count++;
	spin_unlock(&sb_lock);

	if (down_read_trylock(&sb->s_umount)) {
		if (sb->s_root && (sb->s_flags & MS_BORN))
			return true;
		up_read(&sb->s_umount);
	}

	put_super(sb);
	return false;
}
