arch-riscv: Fix the pmp check for pagetable walker

During the page table walking,
MMU will perform the PMP check for each page table page.
However, in the current implementation,
the param:mode used by pmp_Check() is equal to the MMU mode,
which means the page table page has an executable mode
if the target page is executable (during pmp_Check).
As the page table page will never be executable,
the mode for the page table page is either read or write.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-1143

Change-Id: I105f52ef58fe1fbbf7d84c6563e8a8c22cea9ccb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55063
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>

src/arch/riscv/pagetable_walker.cc

@@ -305,7 +305,7 @@ Walker::WalkerState::stepWalk(PacketPtr &write)
     walker->pma->check(read->req);
     // Effective privilege mode for pmp checks for page table
     // walks is S mode according to specs
-    fault = walker->pmp->pmpCheck(read->req, mode,
+    fault = walker->pmp->pmpCheck(read->req, BaseMMU::Read,
                     RiscvISA::PrivilegeMode::PRV_S, tc, entry.vaddr);
 
     if (fault == NoFault) {
@@ -357,7 +357,7 @@ Walker::WalkerState::stepWalk(PacketPtr &write)
                         walker->pma->check(read->req);
 
                         fault = walker->pmp->pmpCheck(read->req,
-                                            mode, pmode, tc, entry.vaddr);
+                                            BaseMMU::Write, pmode, tc, entry.vaddr);
 
                     }
                     // perform step 8 only if pmp checks pass