Copy flags over ifr_union directly rather than via casts through ifr_data.

No functional change in practice. If the sbni driver supported
64-bit big-endian system, this would be an ABI changes, but it is
i386-only. The old version leaked a word of stack on 64-bit systems.

This eliminates the only assignment to ifr_data.

Reviewed by: kib
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14874