powerpc/mm: Improve readability of update_mmu_cache()

The function is used to update the MMU with software PTE. It can
be called by data access exception handler (0x300) or instruction
access exception handler (0x400). If the function is called by
0x400 handler, the local variable @access is set to _PAGE_EXEC
to indicate the software PTE should have that flag set. When the
function is called by 0x300 handler, @access is set to zero.

This improves the readability of the function by replacing if
statements with switch. No logical changes introduced.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mem.c

@@ -491,7 +491,8 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 	 * We don't need to worry about _PAGE_PRESENT here because we are
 	 * called with either mm->page_table_lock held or ptl lock held
 	 */
-	unsigned long access = 0, trap;
+	unsigned long access, trap;
+
 	if (radix_enabled())
 		return;
 
@@ -506,13 +507,19 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 	 *
 	 * We also avoid filling the hash if not coming from a fault
 	 */
-	if (current->thread.regs == NULL)
-		return;
-	trap = TRAP(current->thread.regs);
-	if (trap == 0x400)
-		access |= _PAGE_EXEC;
-	else if (trap != 0x300)
+
+	trap = current->thread.regs ? TRAP(current->thread.regs) : 0UL;
+	switch (trap) {
+	case 0x300:
+		access = 0UL;
+		break;
+	case 0x400:
+		access = _PAGE_EXEC;
+		break;
+	default:
 		return;
+	}
+
 	hash_preload(vma->vm_mm, address, access, trap);
 #endif /* CONFIG_PPC_STD_MMU */
 #if (defined(CONFIG_PPC_BOOK3E_64) || defined(CONFIG_PPC_FSL_BOOK3E)) \