trace/kprobes: Add back warning about offset in return probes

Let's not remove the warning about offsets and return probes when the
offset is invalid.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/20170227115204.00f92846@gandalf.local.home


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>