MIPS: Remove unused task_struct.trap_no field.

It is initialized to zero and only ever read.  Remove it, and pass zero in
its place.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1531/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>