lttng-modules: Fix crash on powerpc64

lttng-modules: Fix crash on powerpc64

Backport a patch to fix the following on powerpc64 ABIv2.

root@qemuppc64:~# lttng create trace_session --live -U net://127.0.0.1
Spawning a session daemon
lttng_kretprobes: loading out-of-tree module taints kernel.
BUG: Unable to handle kernel data access on read at 0xfffffffffffffff8
Faulting instruction address: 0xc0000000001f6fd0
Oops: Kernel access of bad area, sig: 11 [#1]
<snip>

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>