rtsock: fix a stack overflow

struct sockaddr is not sufficient for buffer that can hold any
sockaddr_* structure. struct sockaddr_storage should be used.

Test:
ifconfig epair create
ifconfig epair0a inet6 add 2001:db8::1 up
ndp -s 2001:db8::2 02:86:98:2e:96:0b proxy # this triggers kernel stack overflow

Reviewed by: markj, kp
Differential Revision: https://reviews.freebsd.org/D35188