MIPS: Set trap_no field in thread_struct on exception.

This reverts commit 7281cd2297 and adds
actual functionality to use the field.

arch/mips/include/asm/processor.h

@@ -275,6 +275,7 @@ struct thread_struct {
 	unsigned long cp0_badvaddr;	/* Last user fault */
 	unsigned long cp0_baduaddr;	/* Last kernel fault accessing USEG */
 	unsigned long error_code;
+	unsigned long trap_nr;
 #ifdef CONFIG_CPU_CAVIUM_OCTEON
 	struct octeon_cop2_state cp2 __attribute__ ((__aligned__(128)));
 	struct octeon_cvmseg_state cvmseg __attribute__ ((__aligned__(128)));
@@ -341,6 +342,7 @@ struct thread_struct {
 	.cp0_badvaddr		= 0,				\
 	.cp0_baduaddr		= 0,				\
 	.error_code		= 0,				\
+	.trap_nr		= 0,				\
 	/*							\
 	 * Platform specific cop2 registers(null if no COP2)	\
 	 */							\