ndp(8): increase buffer size in rtsock mode

On a router with many connected devices (~10k+) ndp -an can fail
with ENOMEM because of some additional NDP records were added
between sysctl() buffer size estimate and data fetch calls.

Allocate more space based on size estimate: 1/64 (~2%) of additional
space, but not less that 4 m_rtmsg structures.

Obtained from: Yandex LLC
MFC after: 2 weeks
Sponsored by: Yandex LLC
Differential Revision: https://reviews.freebsd.org/D43956