xfs: fix I_DONTCACHE

Yup, the VFS hoist broke it, and nobody noticed. Bulkstat workloads
make it clear that it doesn't work as it should.

Fixes: dae2f8ed ("fs: Lift XFS_IDONTCACHE to the VFS layer")
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>