fib algo: do not reallocate datapath index for datapath ptr update.

Fib algo uses a per-family array indexed by the fibnum to store
lookup function pointers and per-fib data.

Each algorithm rebuild currently requires re-allocating this array
to support atomic change of two pointers.

As in reality most of the changes actually involve changing only
data pointer, add a shortcut performing in-flight pointer update.

MFC after: 2 weeks