RDS/IB: Remove ib_[header/data]_sge() functions

These functions were to cope with differently ordered
sg entries depending on RDS 3.0 or 3.1+. Now that
we've dropped 3.0 compatibility we no longer need them.

Also, modify usage sites for these to refer to sge[0] or [1]
directly. Reorder code to initialize header sgs first.

Signed-off-by: Andy Grover <andy.grover@oracle.com>