ARC: [ASID] get_new_mmu_context() to conditionally allocate new ASID

ASID allocation changes/1

This patch does 2 things:

(1) get_new_mmu_context() NOW moves mm->ASID to a new value ONLY if it
    was from a prev allocation cycle/generation OR if mm had no ASID
    allocated (vs. before would unconditionally moving to a new ASID)

    Callers desiring unconditional update of ASID, e.g.local_flush_tlb_mm()
    (for parent's address space invalidation at fork) need to first force
    the parent to an unallocated ASID.

(2) get_new_mmu_context() always sets the MMU PID reg with unchanged/new
    ASID value.

The gains are:
- consolidation of all asid alloc logic into get_new_mmu_context()
- avoiding code duplication in switch_mm() for PID reg setting
- Enables future change to fold activate_mm() into switch_mm()

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/mm/tlb.c

@@ -258,13 +258,14 @@ noinline void local_flush_tlb_mm(struct mm_struct *mm)
 		return;
 
 	/*
-	 * Workaround for Android weirdism:
-	 * A binder VMA could end up in a task such that vma->mm != tsk->mm
-	 * old code would cause h/w - s/w ASID to get out of sync
+	 * - Move to a new ASID, but only if the mm is still wired in
+	 *   (Android Binder ended up calling this for vma->mm != tsk->mm,
+	 *    causing h/w - s/w ASID to get out of sync)
+	 * - Also get_new_mmu_context() new implementation allocates a new
+	 *   ASID only if it is not allocated already - so unallocate first
 	 */
-	if (current->mm != mm)
-		destroy_context(mm);
-	else
+	destroy_context(mm);
+	if (current->mm == mm)
 		get_new_mmu_context(mm);
 }