IFCAP_NOMAP indicates that a driver is well behaved with respect to unmapped mbufs. This means that the driver not doing anything grotty with the contents of non-header mbufs on transmit. Specifically, it indicates that the driver is not doing something like calling mtod() and memcpy()'ing packets, and that the driver is using busdma (or sglist) to manage dma, rather than calling pmap_kextract(). Given that iflib fits this definition, and given that it does not pass raw mbufs to drivers, then we should allow it to skip the mb_unmapped_to_ext()
Note that unmapped mbufs have been well tested with iflib when using the ix and ixl drivers.