OpenZFS 7603 - xuio_stat_wbuf_* should be declared (void)

Porting Notes:

• include/sys/dmu.h prototypes were already updated in 0bc8fd7

Authored by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7603
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/99aa8b5
Closes #5586