IB/core: postpone WR initialization during queue drain

No need to initialize completion and WR in case we fail
during QP modification.

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>