xfs: Introduce XFS_DIFLAG2_NREXT64 and associated helpers

xfs: Introduce XFS_DIFLAG2_NREXT64 and associated helpers

Source kernel commit: 9b7d16e34bbebc0398b1dd4f2d64ae6793fdc5ea

This commit adds the new per-inode flag XFS_DIFLAG2_NREXT64 to indicate that
an inode supports 64-bit extent counters. This flag is also enabled by default
on newly created inodes when the corresponding filesystem has large extent
counter feature bit (i.e. XFS_FEAT_NREXT64) set.

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>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>