netinet6: Fix ND link-layer address option layout for IPoIB

RFC 4391 (IP over InfiniBand), section 9.3, lays the ND source/target
link-layer address option out as type, length (3), two reserved zero
octets, then the 20-octet IPoIB link-layer address.
The ND code assumed the Ethernet layout (RFC 4861, section 4.6.1)
everywhere and read/wrote the address directly after the option
header, i.e. two octets early.
The option-length sanity check computes 24 for both layouts for
a 20-octet address, so the mismatch was silent.

PR: 296585
Reviewed by: adrian, pouria
Sponsored by: VersatusHPC
Differential Revision: https://reviews.freebsd.org/D58096