bpf: Fix extable fixup offset.

The prog - start_of_ldx is the offset before the faulting ldx to the location
after it, so this will be used to adjust pt_regs->ip for jumping over it and
continuing, and with old temp it would have been fixed up to the wrong offset,
causing crash.

Fixes: 4c5de127

 ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>