x86/pagewalk: Address MISRA R8.3 violation in guest_walk_tables()

Commit 4c5d78a10d ("x86/pagewalk: Re-implement the pagetable walker")
intentionally renamed guest_walk_tables()'s 'pfec' parameter to 'walk' because
it's not a PageFault Error Code, despite the name of some of the constants
passed in.  Sadly the constants-cleanup I've been meaning to do since then
still hasn't come to pass.

Update the declaration to match, to placate MISRA.

Fixes: 4c5d78a10d ("x86/pagewalk: Re-implement the pagetable walker")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/x86/include/asm/guest_pt.h

@@ -422,7 +422,7 @@ static inline unsigned int guest_walk_to_page_order(const walk_t *gw)
 
 bool
 guest_walk_tables(const struct vcpu *v, struct p2m_domain *p2m,
-                  unsigned long va, walk_t *gw, uint32_t pfec,
+                  unsigned long va, walk_t *gw, uint32_t walk,
                   gfn_t top_gfn, mfn_t top_mfn, void *top_map);
 
 /* Pretty-print the contents of a guest-walk */