ARM: 8834/1: Fix: kprobes: optimized kprobes illegal instruction

commit e46daee5 ("ARM: 8806/1: kprobes: Fix false positive with
FORTIFY_SOURCE") introduced a regression in optimized kprobes. It
triggers "invalid instruction" oopses when using kprobes instrumentation
through lttng and perf. This commit was introduced in kernel v4.20, and
has been backported to stable kernels 4.19 and 4.14.

This crash was also reported by Hongzhi Song on the redhat bugzilla
where the patch was originally introduced.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1639397
Link: https://bugs.lttng.org/issues/1174
Link: https://lore.kernel.org/lkml/342740659.2887.1549307721609.JavaMail.zimbra@efficios.com

Fixes: e46daee5

 ("ARM: 8806/1: kprobes: Fix false positive with FORTIFY_SOURCE")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reported-by: Robert Berger <Robert.Berger@ReliableEmbeddedSystems.com>
Tested-by: Robert Berger <Robert.Berger@ReliableEmbeddedSystems.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Robert Berger <Robert.Berger@ReliableEmbeddedSystems.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: <stable@vger.kernel.org> # v4.14+
Cc: linux-arm-kernel@lists.infradead.org
Cc: patches@armlinux.org.uk
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>