xfs_iflush_fork() never returns an error. Mark it void and clean up the
code calling it that checks for errors.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30827a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>