Dpdk Backend: Fix incorrect field extraction arithmetic

[usha1830]

when multiple non-byte aligned header fields are combined into single byte aligned field. The references of these fields are updated with incorrect slice operation.

For ex: when

 header ipv4_t {
                            ...
                            bit<3>  flags;
                            bit<13> fragOffset;
                            ...
                        }

is converted into

                  header ipv4_t {
                           ...
                           bit<16>  flags_fragOffset;
                           ...
                       }

flags field should be converted to flags_fragOffset[15:13]
fragOffset should be converted to flags_fragOffset[12:0]

But currently the conversion happens like this
flags field is converted to flags_fragOffset[2:0]
fragOffset is converted to flags_fragOffset[15:3]

Fixes #4077

[mihaibudiu]

how come this bug wasn't caught?

[jafingerhut]

It was caught. That is why there was a bug report about it.

Do you mean, why wasn't it caught earlier than Jul 2023 when I reported it?

Probably at least in part because there are no automated CI tests that pass data packets through the P4 DPDK that would have caught this. Fabian Ruffy has tried to create some such tests, but he has had difficulty making them work consistently enough to use for CI purposes on this repository.

[usha1830]

And Perhaps no user tried accessing the individual header fields of the combined fields. Some of the early users may be combining these fields by themselves in the P4 program and thereby handling these extractions also in the program itself.
Thanks to Andy who caught this!!