xfs: reinstate the old i_version counter as STATX_CHANGE_COOKIE

The handling of STATX_CHANGE_COOKIE was moved into generic_fillattr in
commit 0d72b928 (fs: pass the request_mask to generic_fillattr), but
we didn't account for the fact that xfs doesn't call generic_fillattr at
all.

Make XFS report its i_version as the STATX_CHANGE_COOKIE.

Fixes: 0d72b928 (fs: pass the request_mask to generic_fillattr)
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>