bpf/verifier: Add spi variable to check_stack_write()

Extracted from commit dc503a8ad984 "bpf/verifier: track liveness for
pruning".

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>