xfs: change some error-less functions to void types

There are several functions which have no opportunity to return
an error, and don't contain any ASSERTs which could be argued
to be better constructed as error cases.  So, make them voids
to simplify the callers.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>