nfp: bpf: allow stack accesses via modified stack registers

As long as the verifier tells us the stack offset exactly we
can render the LMEM reads quite easily.  Simply make sure that
the offset is constant for a given instruction and add it to
the instruction's offset.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>