ndp: Move the global variable repeat into struct ndp_opts

An upcoming fix for the netlink based implementation requires this
refactoring.

No functional change intended.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47233

(cherry picked from commit 12c04071cf18d1b00ffcc7058fb01f4393163ede)