Annotated dprintf as printf-like

ZFS loves using %llu for uint64_t, but that requires a cast to not 
be noisy - which is even done in many, though not all, places.
Also a couple places used %u for uint64_t, which were promoted
to %llu. 

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12233

include/sys/zfs_debug.h

@@ -61,7 +61,7 @@ extern int zfs_dbgmsg_enable;
 extern void __set_error(const char *file, const char *func, int line, int err);
 extern void __zfs_dbgmsg(char *buf);
 extern void __dprintf(boolean_t dprint, const char *file, const char *func,
-    int line, const char *fmt, ...);
+    int line, const char *fmt, ...)  __attribute__((format(printf, 5, 6)));
 
 /*
  * Some general principles for using zfs_dbgmsg():