linuxkpi: print stack trace in WARN_ON macros

Reviewed by:	hselasky, kib
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D24779

sys/compat/linuxkpi/common/include/linux/kernel.h

@@ -107,6 +107,7 @@
       if (__ret) {						\
 		printf("WARNING %s failed at %s:%d\n",		\
 		    __stringify(cond), __FILE__, __LINE__);	\
+		linux_dump_stack();				\
       }								\
       unlikely(__ret);						\
 })
@@ -120,6 +121,7 @@
 		__warn_on_once = 1;				\
 		printf("WARNING %s failed at %s:%d\n",		\
 		    __stringify(cond), __FILE__, __LINE__);	\
+		linux_dump_stack();				\
       }								\
       unlikely(__ret);						\
 })