fboss/agent/ndp/IPv6RouteAdvertiser.cpp: avoid shadowing warnings
fboss/agent/ndp/IPv6RouteAdvertiser.cpp: avoid shadowing warnings

Summary:
Fix warnings exposed by gcc-4.9.x's -Wshadow-compatible-local
I plan to enable this for all of fbcode, soon.
See https://fburl.com/bad-shadow for justification.

Rename inner "cursor" to "cur".

This avoids the following errors:

  fboss/agent/ndp/IPv6RouteAdvertiser.cpp:245:51: error: declaration of 'folly::io::RWPrivateCursor* cursor' shadows a parameter [-Werror=shadow-compatible-local]
  fboss/agent/ndp/IPv6RouteAdvertiser.cpp:212:33: error: shadowed declaration is here [-Werror=shadow-compatible-local]

Differential Revision: D4656615

fbshipit-source-id: a9f59704a56feefc834c819637e524077ac2e42e

peterlei/fboss,peterlei/fboss,peterlei/fboss,peterlei/fboss