[XFS] kill xfs_getattr

It's currently used by the ACL code to read di_mode/di_uid, but these are
simple 32bit scalar values we can just read directly without locking.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30897a

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