xen/arm: p2m: Don't check the return of p2m_get_root_pointer() with BUG_ON()

xen/arm: p2m: Don't check the return of p2m_get_root_pointer() with BUG_ON()

It turns out that the BUG_ON() was actually reachable with well-crafted
hypercalls. The BUG_ON() is here to prevent catch logical error, so
crashing Xen is a bit over the top.

While all the holes should now be fixed, it would be better to downgrade
the BUG_ON() to something less fatal to prevent any more DoS.

The BUG_ON() in p2m_get_entry() is now replaced by ASSERT_UNREACHABLE()
to catch mistake in debug build and return INVALID_MFN for production
build. The interface also requires to set page_order to give an idea of
the size of "hole". So 'level' is now set so we report a hole of size of
the an entry of the root page-table. This stays inline with what happen
when the GFN is higher than p2m->max_mapped_gfn.

The BUG_ON() in p2m_resolve_translation_fault() is now replaced by
ASSERT_UNREACHABLE() to catch mistake in debug build and just report a
fault for producion build.

This is part of XSA-301.

Reported-by: Julien Grall <Julien.Grall@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>