[iwm] Plug memory leak, and fix error handling in iwm_rx_addbuf().

[imre_vdsz.com]

Summary

• When allocating a new mbuf or bus_dmamap_load()-ing it fails, we can just keep the old mbuf since we are dropping that packet anyway.

• Instead of doing bus_dmamap_create() and bus_dmamap_destroy() all the time, create an extra bus_dmamap_t which we can use to safely try bus_dmamap_load()-ing the new mbuf. On success we just swap the spare bus_dmamap_t with the data->map of that ring entry.

Obtained from: DragonFlyBSD cc440b26818b5dfdd9af504d71c1b0e6522b53ef

Test Plan

Tested with Intel AC7260, verified with vmstat -m that new kernel no
longer visibly leaks memory from the M_DEVBUF malloc type.
Before, leakage was 1KB every few seconds while ping(8)-ing over the wlan
connection.

[imre_vdsz.com]

Avoid doing bus_dmamap_create()/_destroy() all the time in iwm_rx_addbuf()