btrfs: only invalidate btree inode pages after all ebs are released

In close_ctree(), we call invalidate_inode_pages2() to invalidate all
pages from btree inode.

But the problem is, it never returns 0, but always -EBUSY.

The problem is that we are still holding all the essential tree root
nodes, thus pages holding those tree blocks can not be invalidated thus
invalidate_inode_pages2() always returns -EBUSY.

This is also against the error cleanup path of open_ctree(), which
properly frees all root pointers before calling invalidate_inode_pages().

So fix the order by delaying invalidate_inode_pages2() until we have
freed all root pointers.

Reviewed-by: Anand Jain <asj@kernel.org>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>