Fix leaks caused by ifc_nhwtxqs never being initialized

[shurd]

Summary

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

Test Plan

Test with ixgbe, bnxt, and e1000 including use as a module.
Load/unload the module multiple times.

[pkelsey]

Looks good to me.

Style-wise, would it be better to set up an sctx local at the top of these routines and then dereference that to avoid the double-dereference noise in the code?

[shurd]

Address feedback.

[shurd]

In D17979#384108, @pkelsey wrote:

Style-wise, would it be better to set up an sctx local at the top of these routines and then dereference that to avoid the double-dereference noise in the code?

I'm not sure it's any better, but I don't mind doing it.