Fix leaks caused by ifc_nhwtxqs never being initialized

r333502 removed initialization of ifc_nhwtxqs, and it's not clear
there's a need to copy it into the struct iflib_ctx at all. Use
ctx->ifc_sctx->isc_ntxqs instead.

Further, iflib_stop() did not clear the last ring in the case where
isc_nfl != isc_nrxqs (such as when IFLIB_HAS_RXCQ is set). Use
ctx->ifc_sctx->isc_nrxqs here instead of isc_nfl.

Reported by: pkelsey
Reviewed by: pkelsey
MFC after: 3 days
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D17979