net: fix compilation when busy poll is not enabled

MIN_NAPI_ID is used in various places outside of
CONFIG_NET_RX_BUSY_POLL wrapping, so when it's not set
we run into build errors such as:

  net/core/dev.c: In function 'dev_get_by_napi_id':
  net/core/dev.c:886:16: error: ‘MIN_NAPI_ID’ undeclared (first use in this function)
    if (napi_id < MIN_NAPI_ID)
                  ^~~~~~~~~~~

Thus, have MIN_NAPI_ID always defined to fix these errors.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/busy_poll.h

@@ -29,18 +29,18 @@
 #include <linux/sched/signal.h>
 #include <net/ip.h>
 
-#ifdef CONFIG_NET_RX_BUSY_POLL
-
-struct napi_struct;
-extern unsigned int sysctl_net_busy_read __read_mostly;
-extern unsigned int sysctl_net_busy_poll __read_mostly;
-
 /*		0 - Reserved to indicate value not set
  *     1..NR_CPUS - Reserved for sender_cpu
  *  NR_CPUS+1..~0 - Region available for NAPI IDs
  */
 #define MIN_NAPI_ID ((unsigned int)(NR_CPUS + 1))
 
+#ifdef CONFIG_NET_RX_BUSY_POLL
+
+struct napi_struct;
+extern unsigned int sysctl_net_busy_read __read_mostly;
+extern unsigned int sysctl_net_busy_poll __read_mostly;
+
 static inline bool net_busy_loop_on(void)
 {
 	return sysctl_net_busy_poll;