nfs: include xattr.h from fs/nfs/nfs3proc.c

nfs: include xattr.h from fs/nfs/nfs3proc.c

fs/nfs/nfs3proc.c is making use of xattr but was getting linux/xattr.h
indirectly through linux/cgroup.h, which will soon drop the inclusion
of xattr.h.  Explicitly include linux/xattr.h from nfs3proc.c so that
compilation doesn't fail when linux/cgroup.h drops linux/xattr.h.

As the following cgroup changes will depend on these changes, it
probably would be easier to route this through cgroup branch.  Would
that be okay?

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: linux-nfs@vger.kernel.org