After r275136, kernel compilation failed with RSS option.

/usr/src/sys/dev/e1000/if_igb.c:2883:6: error: variable 'queues' is
      uninitialized when used here [-Werror,-Wuninitialized]
        if (queues > rss_getnumbuckets())
            ^~~~~~
/usr/src/sys/dev/e1000/if_igb.c:2839:24: note: initialize the variable 'queues'
      to silence this warning
        int             bar, want, queues, msgs, maxqueues;
                                         ^
                                          = 0

In igb_setup_msix(), if RSS is defined, 'queues' is used before substituted.

2881	#ifdef  RSS
2882	        /* If we're doing RSS, clamp at the number of RSS buckets */
2883	        if (queues > rss_getnumbuckets())
2884	                queues = rss_getnumbuckets();
2885	#endif
2886	
2887	        if (n_queues != 0) {
2888	                queues = n_queues;