filter: use unsigned int to silence static checker warning

This is just a cleanup.

My testing version of Smatch warns about this:
net/core/filter.c +380 check_load_and_stores(6)
	warn: check 'flen' for negative values

flen comes from the user.  We try to clamp the values here between 1
and BPF_MAXINSNS but the clamp doesn't work because it could be
negative.  This is a bug, but it's not exploitable.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -155,7 +155,7 @@ extern unsigned int sk_run_filter(const struct sk_buff *skb,
 				  const struct sock_filter *filter);
 extern int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk);
 extern int sk_detach_filter(struct sock *sk);
-extern int sk_chk_filter(struct sock_filter *filter, int flen);
+extern int sk_chk_filter(struct sock_filter *filter, unsigned int flen);
 
 #ifdef CONFIG_BPF_JIT
 extern void bpf_jit_compile(struct sk_filter *fp);

net/core/filter.c

@@ -436,7 +436,7 @@ static int check_load_and_stores(struct sock_filter *filter, int flen)
  *
  * Returns 0 if the rule set is legal or -EINVAL if not.
  */
-int sk_chk_filter(struct sock_filter *filter, int flen)
+int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
 {
 	/*
 	 * Valid instructions are initialized to non-0.