uprobes/tracing: Kill the pointless task_pt_regs() calls

uprobe_trace_func() and uprobe_perf_func() do not need task_pt_regs(),
we already have "struct pt_regs *regs".

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