bpf: Fix compilation warning in __bpf_lru_list_rotate_inactive

gcc-6.2.1 gives the following warning:
kernel/bpf/bpf_lru_list.c: In function ‘__bpf_lru_list_rotate_inactive.isra.3’:
kernel/bpf/bpf_lru_list.c:201:28: warning: ‘next’ may be used uninitialized in this function [-Wmaybe-uninitialized]

The "next" is currently initialized in the while() loop which must have >=1
iterations.

This patch initializes next to get rid of the compiler warning.

Fixes: 3a08c2fd ("bpf: LRU List")
Reported-by: NDavid Miller <davem@davemloft.net>
Signed-off-by: NMartin KaFai Lau <kafai@fb.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

kernel/bpf/bpf_lru_list.c

@@ -170,7 +170,7 @@ static void __bpf_lru_list_rotate_inactive(struct bpf_lru *lru,
 					   struct bpf_lru_list *l)
 {
 	struct list_head *inactive = &l->lists[BPF_LRU_LIST_T_INACTIVE];
-	struct list_head *cur, *next, *last;
+	struct list_head *cur, *last, *next = inactive;
 	struct bpf_lru_node *node;
 	unsigned int i = 0;