x86/mm: Make p2m lookups fully synchronized wrt modifications

x86/mm: Make p2m lookups fully synchronized wrt modifications

We achieve this by locking/unlocking the global p2m_lock in get/put_gfn.

The lock is always taken recursively, as there are many paths that
call get_gfn, and later, make another attempt at grabbing the p2m_lock.

The lock is not taken for shadow lookups. We believe there are no problems
remaining for synchronized p2m+shadow paging, but we are not enabling this
combination due to lack of testing. Unlocked shadow p2m access are tolerable as
long as shadows do not gain support for paging or sharing.

HAP (EPT) lookups and all modifications do take the lock.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>