MIPS: fix uretprobe implementation

arch_uretprobe_hijack_return_addr should replace the return address for
a call with a trampoline address.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Fixes: 40e084a5

 ('MIPS: Add uprobes support.')
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14298/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>