nfp: bpf: add helper for validating stack pointers

nfp: bpf: add helper for validating stack pointers

Our implementation has restriction on stack pointers for function
calls.  Move the common checks into a helper for reuse.  The state
has to be encapsulated into a structure to support parameters
other than BPF_REG_2.

Signed-off-by: Jakub Kicinski <[email protected]>
Reviewed-by: Quentin Monnet <[email protected]>
Reviewed-by: Jiong Wang <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>