trace: reenable preemption if we modify the ip

Things got moved around between the original bpf_override_return patches
and the final version, and now the ftrace kprobe dispatcher assumes if
you modified the ip that you also enabled preemption.  Make a comment of
this and enable preemption, this fixes the lockdep splat that happened
when using this feature.

Fixes: 9802d865 ("bpf: add a bpf_override_function helper")
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>