MIPS: mm: Add set_cpu_context() for ASID assignments

When we gain MMID support we'll be storing MMIDs as atomic64_t values
and accessing them via atomic64_* functions. This necessitates that we
don't use cpu_context() as the left hand side of an assignment, ie. as a
modifiable lvalue. In preparation for this introduce a new
set_cpu_context() function & replace all assignments with cpu_context()
on their left hand side with an equivalent call to set_cpu_context().

To enforce that cpu_context() should not be used for assignments, we
rewrite it as a static inline function.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

arch/mips/mm/context.c

@@ -15,7 +15,8 @@ void get_new_mmu_context(struct mm_struct *mm)
 		local_flush_tlb_all();	/* start new asid cycle */
 	}
 
-	cpu_context(cpu, mm) = asid_cache(cpu) = asid;
+	set_cpu_context(cpu, mm, asid);
+	asid_cache(cpu) = asid;
 }
 
 void check_mmu_context(struct mm_struct *mm)