nios2: force update_mmu_cache on spurious tlb-permission--related pagefaults

NIOS2 uses a software-managed TLB for virtual address translation. To
flush a cache line, the original mapping is replaced by one to physical
address 0x0 with no permissions (rwx mapped to 0) set. This can lead to
TLB-permission--related traps when such a nominally flushed entry is
encountered as a mapping for an otherwise valid virtual address within a
process (e.g. due to an MMU-PID-namespace rollover that previously
flushed the complete TLB including entries of existing, running
processes).

The default ptep_set_access_flags implementation from mm/pgtable-generic.c
only forces a TLB-update when the page-table entry has changed within the
page table:

	/*
	 * [...] We return whether the PTE actually changed, which in turn
	 * instructs the caller to do things like update__mmu_cache. [...]
	 */
	int ptep_set_access_flags(struct vm_area_struct *vma,
				  unsigned long address, pte_t *ptep,
				  pte_t entry, int dirty)
	{
		int changed = !pte_same(*ptep, entry);
		if (changed) {
			set_pte_at(vma->vm_mm, address, ptep, entry);
			flush_tlb_fix_spurious_fault(vma, address);
		}
		return changed;
	}

However, no cross-referencing with the TLB-state occurs, so the
flushing-induced pseudo entries that are responsible for the pagefault
in the first place are never pre-empted from TLB on this code path.

This commit fixes this behaviour by always requesting a TLB-update in
this part of the pagefault handling, fixing spurious page-faults on the
way. The handling is a straightforward port of the logic from the MIPS
architecture via an arch-specific ptep_set_access_flags function ported
from arch/mips/include/asm/pgtable.h.

Signed-off-by: Simon Schuster <schuster.simon@siemens-energy.com>
Signed-off-by: Andreas Oetken <andreas.oetken@siemens-energy.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>