RDS: Assume recv->r_frag is always NULL in refill_one()

refill_one() should never be called on a recv struct that
doesn't need a new r_frag allocated. Add a WARN and remove
conditional around r_frag alloc code.

Also, add a comment to explain why r_ibinc may or may not
need refilling.

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