Enable compiler to typecheck logging

Annotate spa logging declarations with printflike
Workaround gcc bug (non disable-able warning) by
replacing "" with " "

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9316

include/os/linux/spl/sys/debug.h

@@ -50,6 +50,8 @@
 /*
  * Common DEBUG functionality.
  */
+#define	__printflike(a, b)	__printf(a, b)
+
 int spl_panic(const char *file, const char *func, int line,
     const char *fmt, ...);
 void spl_dumpstack(void);