ndp: Use nitems(foo) instead of sizeof(foo)/sizeof(foo[0])

Pull Request: https://github.com/freebsd/freebsd-src/pull/888
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>

(cherry picked from commit bccbb2fc4bce6c3efc1ba93f222de320db8e020a)
(cherry picked from commit 635037210ea3e644c9d76b9627323f5ddf5dc5f5)