netpoll: don't require irqs disabled in rt kernels

write_msg(netconsole.c:836) calls netpoll_send_udp after a call to
spin_lock_irqsave, which normally disables interrupts; but in PREEMPT_RT
this call just locks an rt_mutex without disabling irqs. In this case,
netpoll_send_udp is called with interrupts enabled.
Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -36,6 +36,7 @@
 #include <net/ip6_checksum.h>
 #include <asm/unaligned.h>
 #include <trace/events/napi.h>
+#include <linux/kconfig.h>
 
 /*
  * We maintain a small pool of fully-sized skbs, to make sure the
@@ -389,7 +390,8 @@ void netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 	static atomic_t ip_ident;
 	struct ipv6hdr *ip6h;
 
-	WARN_ON_ONCE(!irqs_disabled());
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+		WARN_ON_ONCE(!irqs_disabled());
 
 	udp_len = len + sizeof(*udph);
 	if (np->ipv6)