xfs: Enable bulkstat ioctl to support 64-bit per-inode extent counters

The following changes are made to enable userspace to obtain 64-bit extent
counters,
1. Carve out a new 64-bit field xfs_bulkstat->bs_extents64 from
   xfs_bulkstat->bs_pad[] to hold 64-bit extent counter.
2. Define the new flag XFS_BULK_IREQ_BULKSTAT for userspace to indicate that
   it is capable of receiving 64-bit extent counters.

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