[XFS] Use the generic xattr methods.

Use the generic set, get and removexattr methods and supply the s_xattr
array with fine-grained handlers. All XFS/Linux highlevel attr handling is
rewritten from scratch and placed into fs/xfs/linux-2.6/xfs_xattr.c so
that it's separated from the generic low-level code.

SGI-PV: 982343

SGI-Modid: xfs-linux-melb:xfs-kern:31234a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>