netinet: correct SIOCDIFADDR{,_IN6} calls to use {,in6_}ifreq

The SIOCDIFADDR{,_IN6} ioctls take an ifreq structure object, not an
ifaliasreq/in_aliasreq/in6_aliasreq structure object, as their argument.
As opposed to ifaliasreq/in_aliasreq/in6_aliasreq used by
SIOCAIFADDR{,_IN6}, the ifreq/in6_ifreq structures used by the
SIOCDIFADDR{,_IN6} ioctls do not include a separate field for a
broadcast address and other values required to add an address to a
network interface with SIOCAIFADDR{,_IN6}.

Whilst this issue is not specific to CHERI-extended architectures, it
was first observed on CheriBSD running on Arm Morello. For example,
incorrect calls using the in6_aliasreq object result in CHERI capability
violations. A pointer to the ifra_addr field in in6_aliasreq cast to the
ifru_addr union member of in6_ifreq results in bounds being set to the
union's larger size. Such bounds exceed the bounds of of in6_aliasreq
object and the bounds-setting instruction clears a tag of the object's
capability.

Reviewed by: brooks, kp, oshogbo
Accepted by: oshogbo (mentor)
Reported by: CHERI
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D46016