netmap: get rid of save_if_input for emulated adapters

The save_if_input function pointer was meant to save the previous
value of ifp->if_input before replacing it with the emulated
adapter hook.
However, the same pointer value is already stored in the if_input
field of the netmap_adapter struct, to be used for host TX ring processing.

Reuse the netmap_adapter if_input field to simplify the code
and save some space.

MFC after: 14 days

(cherry picked from commit 6c9fe357435db6408ec6b01221b9ff9e7e9b0ff1)