bpf/verifier: Pass instruction index to check_mem_access() and check_xadd()

Extracted from commit 31fd85816dbe "bpf: permits narrower load from
bpf program context fields".

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>