[RFC] simplifying the "is it mounted" checks for nilfs2

[Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>]

Do you see any problems with just giving nilfs ->drop_inode() instance
like this:
int nilfs2_drop_inode(struct inode *inode)
{
	if (unlikely(inode->i_ino == NILFS_ROOT_INO))
		return true;
	return generic_drop_inode(inode);
}
Then nilfs_root will live exactly until we unmount the damn thing, giving us
int nilfs_checkpoint_is_mounted(struct super_block *sb, __u64 cno)
{
        struct the_nilfs *nilfs = sb->s_fs_info;

        if (cno < 0 || cno > nilfs->ns_cno)
                return false;

        if (cno >= nilfs_last_cno(nilfs))
                return true;    /* protect recent checkpoints */

        return nilfs_lookup_root(nilfs, cno) != NULL;
}
while the logics in nilfs_mount becoming simply
	busy = d_count(s->s_root) > 1;
and we don't need to bother with nilfs_try_to_shrink_tree() at all.

I might be missing something, but it looks like that would work...
Comments?
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