MINOR: stktable: fix potential build issue in smp_to_stkey (2nd try)

MINOR: stktable: fix potential build issue in smp_to_stkey (2nd try)

As discussed in GH #2838, the previous fix f399dbf
("MINOR: stktable: fix potential build issue in smp_to_stkey") which
attempted to remove conversion ambiguity and prevent build warning proved
to be insufficient.

This time, we implement Willy's suggestion, which is to use an union to
perform the conversion.

Hopefully this should fix GH #2838. If that's the case (and only in that
case), then this patch may be backported with f399dbf (else the patch
won't apply) anywhere b59d1fd ("BUG/MINOR: stktable: fix big-endian
compatiblity in smp_to_stkey()") was backported.