xfs: use a separate slab cache for deferred xattr work state

Source kernel commit: e2c78949b641d34cb4051b32c2fa5e03a4bfef35

Create a separate slab cache for struct xfs_attr_item objects, since we
can pack the (104-byte) intent items more tightly than we can with the
general slab cache objects.  On x86, this means 39 intents per memory
page instead of 32.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>