uprobes: Make arch_uprobe_task->saved_trap_nr "unsigned int"

Make arch_uprobe_task->saved_trap_nr "unsigned int" and move it down
after ->saved_scratch_register, this changes sizeof() from 24 to 16.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>