libfrog: introduce xfs_fd to wrap an fd to a file on an xfs filesystem

libfrog: introduce xfs_fd to wrap an fd to a file on an xfs filesystem

Introduce a new "xfs_fd" context structure where we can store a file
descriptor and all the runtime fs context (geometry, which ioctls work,
etc.) that goes with it.  We're going to create wrappers for the
bulkstat and inumbers ioctls in subsequent patches; and when we
introduce the v5 bulkstat/inumbers ioctls we'll need all that context to
downgrade gracefully on old kernels.  Start the transition by adopting
xfs_fd natively in scrub.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>