kni: remove needless casts

kni: remove needless casts

This fixes cases in KNI where kernel allocation function return value
is needlessly casted.

Detected with coccinelle:
lib/librte_eal/linuxapp/kni/ethtool/igb/igb_main.c:3181:25-28:
	WARNING: casting value returned by memory allocation function to (u32 *) is useless.
lib/librte_eal/linuxapp/kni/kni_vhost.c:690:9-28:
	WARNING: casting value returned by memory allocation function to (struct rte_kni_fifo *) is useless.
lib/librte_eal/linuxapp/kni/kni_vhost.c:684:13-27:
	WARNING: casting value returned by memory allocation function to (struct sk_buff *) is useless

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>