[XFS] split xfs_ioc_xattr

The three subcases of xfs_ioc_xattr don't share any semantics and almost
no code, so split it into three separate helpers.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30709a

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