MIPS: Set trap_no field in thread_struct on exception.

This reverts commit 7281cd22973008a782860e48ed8d85d00204168c and adds
actual functionality to use the field.

diff --cc arch/mips/include/asm/processor.h
Simple merge

diff --cc arch/mips/kernel/asm-offsets.c
Simple merge

diff --cc arch/mips/kernel/traps.c
Simple merge

diff --cc arch/mips/mm/fault.c
Simple merge