radix tree test suite: Reduce kernel.h

Many of the definitions in the radix-tree kernel.h are redundant with
others in tools/include, or are no longer used, such as panic().
Move the definition of __init to init.h and in_interrupt() to preempt.h

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

tools/testing/radix-tree/linux/preempt.h

@@ -1,4 +1,14 @@
+#ifndef __LINUX_PREEMPT_H
+#define __LINUX_PREEMPT_H
+
 extern int preempt_count;
 
 #define preempt_disable()	uatomic_inc(&preempt_count)
 #define preempt_enable()	uatomic_dec(&preempt_count)
+
+static inline int in_interrupt(void)
+{
+	return 0;
+}
+
+#endif /* __LINUX_PREEMPT_H */