xfs: fix check on struct_version for versions 4 or greater

It appears that the check for versions 4 or more is incorrect and is
off-by-one. Fix this.

Detected by CoverityScan, CID#1463775 ("Logically dead code")

Fixes: ac503a4c ("xfs: refactor the geometry structure filling function")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>