dhclient: correct struct ifreq allocation

ioctl commands such as BIOCSETIF take a struct ifreq and due to
FreeBSD's ioctl implementation copy exactly sizeof(struct ifreq) bytes
in so allocate that much space.

The over-allocaton was harmless, but useless.

Reported by: def
Fixes: e2dc8d789f68a dhclient: do not add 0.0.0.0 interface alias.
Sponsored by: DARPA, AFRL

Reviewed by: def
Differential Revision: https://reviews.freebsd.org/D45769