x86/p2m-pt: simplify p2m_next_level()

x86/p2m-pt: simplify p2m_next_level()

Calculate entry PFN and flags just once. Convert the two successive
main if()-s to and if/else-if chain. Restrict variable scope where
reasonable. Take the opportunity and also make the induction variable
unsigned.

This at once fixes excessive permissions granted in the 2M PTEs
resulting from splitting a 1G one - original permissions should be
inherited instead. This is not a security issue only because all of
this takes no effect anyway, as iommu_hap_pt_share is always false on
AMD systems for all supported branches.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>