xfs: add more statics & drop some unused functions

A lot more functions could be made static, but they need
forward declarations; this does some easy ones, and also
found a few unused functions in the process.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Felix Blyakher <felixb@sgi.com>