xfs: several xattr functions can be void

There are a handful of xattr functions which now return
nothing but zero.  They can be made void, chased through calling
functions, and error handling etc can be removed.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>