xfs: don't check agf_btreeblks on pre-lazysbcount filesystems

The AGF free space btree block counter wasn't added until the
lazysbcount feature was added to XFS midway through the life of the V4
format, so ignore the field when checking.  Online AGF repair requires
rmapbt, so it doesn't need the feature check.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>