iflib: netmap: don't increment ifl_cidx on the wrong free list

Netmap only uses free list 0 to keep it consistent with its
one-to-one mapping between each netmap ring and a device RX
(or TX) queue.
However, the current iflib_netmap_rxsync() routine was
mistakenly updating the ifl_cidx field of both free lists.

PR: 248494
MFC after: 2 weeks