nx-match: Check prerequisites for ICMPv6 before outputting subfields.

nx-match: Check prerequisites for ICMPv6 before outputting subfields.

nicira-ext.h documents that NXM_NX_ND_TARGET requires ND_NEIGHBOR_SOLICIT
or ND_NEIGHBOR_ADVERT, that NXM_NX_ND_SLL requires ND_NEIGHBOR_SOLICIT,
and that NXM_NX_ND_TLL requires ND_NEIGHBOR_ADVERT, but nx_put_match()
would add them to the match regardless of whether these prerequisites were
satisfied.  On the other side, nx_pull_match() did check the prerequisites,
so this was a case where OVS could output flows that it would refused to
parse.  This fixes the problem.

Found by inspection.