x86/pagewalk: Remove opt_allow_superpage check from guest_can_use_l2_superpages()

x86/pagewalk: Remove opt_allow_superpage check from guest_can_use_l2_superpages()

The purpose of guest_walk_tables() is to match the behaviour of real hardware.

A PV guest can have 2M superpages in its pagetables, via the M2P (and for dom0
via the initial P2M), even if the guest isn't permitted to create arbitrary 2M
superpage mappings.

guest_can_use_l2_superpages() checking opt_allow_superpage is a piece of PV
guest policy enforcement, rather than its intended purpose of meaning "would
hardware tolerate finding an L2 superpage with these control settings?"

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>