xfs_db: trash the block at the top of the cursor stack

xfs_db: trash the block at the top of the cursor stack

Add a new -z option to blocktrash to make it trash the block that's at
the top of the stack, so that we can perform targeted fuzzing.  While
we're at it, prevent fuzzing off the end of the buffer and add a -o
parameter so that we can specify an offset to start fuzzing from.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>