Remove sdt.h

It's mostly a noop on ZoL and it conflicts with platforms that 
support dtrace.  Remove this header to resolve the conflict.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9497

lib/libspl/include/sys/trace_defs.h

@@ -1 +1,24 @@
 /* Here to keep the libspl build happy */
+
+#ifndef _LIBSPL_ZFS_TRACE_H
+#define	_LIBSPL__ZFS_TRACE_H
+
+/*
+ * The set-error SDT probe is extra static, in that we declare its fake
+ * function literally, rather than with the DTRACE_PROBE1() macro.  This is
+ * necessary so that SET_ERROR() can evaluate to a value, which wouldn't
+ * be possible if it required multiple statements (to declare the function
+ * and then call it).
+ *
+ * SET_ERROR() uses the comma operator so that it can be used without much
+ * additional code.  For example, "return (EINVAL);" becomes
+ * "return (SET_ERROR(EINVAL));".  Note that the argument will be evaluated
+ * twice, so it should not have side effects (e.g. something like:
+ * "return (SET_ERROR(log_error(EINVAL, info)));" would log the error twice).
+ */
+#undef SET_ERROR
+#define	SET_ERROR(err) \
+	(__set_error(__FILE__, __func__, __LINE__, err), err)
+
+
+#endif