net: iflib: sync isc_capenable to if_capenable

On SIOCSIFCAP, some bits in ifp->if_capenable may be toggled.
When this happens, apply the same change to isc_capenable, which
is the iflib private copy of if_capenable (for a subset of the
IFCAP_* bits). In this way the iflib drivers can check the bits
using isc_capenable rather than if_capenable. This is convenient
because the latter access requires an additional indirection
through the ifp, and it is also less likely to be in cache.

PR:		260068
Reviewed by:	kbowling, gallatin
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D33156

sys/net/iflib.c

@@ -4449,6 +4449,7 @@ iflib_if_ioctl(if_t ifp, u_long command, caddr_t data)
 				iflib_stop(ctx);
 			STATE_LOCK(ctx);
 			if_togglecapenable(ifp, setmask);
+			ctx->ifc_softc_ctx.isc_capenable ^= setmask;
 			STATE_UNLOCK(ctx);
 			if (bits & IFF_DRV_RUNNING && setmask & ~IFCAP_WOL)
 				iflib_init_locked(ctx);