xfs: allow bulkstat_single of special inodes

Create a new ireq flag (for single bulkstats) that enables userspace to
ask us for a special inode number instead of interpreting @ino as a
literal inode number.  This enables us to query the root inode easily.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>