netmap: Stop printing a line to the dmesg in netmap_init()

netmap is compiled into the kernel by default so initialization was
always reported, and netmap uses a formatting convention not used in the
rest of the kernel.

Reviewed by:	vmaffione
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29099

sys/dev/netmap/netmap.c

@@ -4347,7 +4347,9 @@ netmap_init(void)
 	if (error)
 		goto fail;
 
+#if !defined(__FreeBSD__) || defined(KLD_MODULE)
 	nm_prinf("netmap: loaded module");
+#endif
 	return (0);
 fail:
 	netmap_fini();