x86/mm: fix invalid unlinking of nested p2m tables

x86/mm: fix invalid unlinking of nested p2m tables

Commit 90805dc (c/s 26387:4056e5a3d815) ("EPT: Make ept data stucture or
operations neutral") makes nested p2m tables be unlinked from the host
p2m table before their destruction (in p2m_teardown_nestedp2m).
However, by this time the host p2m table has already been torn down,
leading to a possible race condition where another allocation between
the two kinds of table being torn down can lead to a linked list
assertion with debug=y builds or memory corruption on debug=n ones.

Fix by swapping the order the two kinds of table are torn down in. While
at it, remove the condition in p2m_final_teardown, as it is already
checked identically in p2m_teardown_hostp2m itself.

Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Tim Deegan <tim@xen.org>