xfs: Introduce XFS_FSOP_GEOM_FLAGS_NREXT64

XFS_FSOP_GEOM_FLAGS_NREXT64 indicates that the current filesystem instance
supports 64-bit per-inode extent counters.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>