OPTIM: ebtree: make ebmb_insert_prefix() keep a copy the new node's pfx

OPTIM: ebtree: make ebmb_insert_prefix() keep a copy the new node's pfx

looking at a perf profile while loading a conf with a huge map, it
appeared that there was a hot spot on the access to the new node's
prefix, which is unexpectedly being reloaded for each visited node
during the tree descent. Better keep a copy of it because with large
trees that don't fit into the L3 cache the memory bandwidth is scarce.
Doing so reduces the load time from 8.0 to 7.5 seconds.