mkfs: Add option to create filesystem with large extent counters

mkfs: Add option to create filesystem with large extent counters

Enabling nrext64 option on mkfs.xfs command line extends the maximum values of
inode data and attr fork extent counters to 2^48 - 1 and 2^32 - 1
respectively.  This also sets the XFS_SB_FEAT_INCOMPAT_NREXT64 incompat flag
on the superblock preventing older kernels from mounting such a filesystem.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>