IB/mlx4: Use kzalloc() for new QPs so flags are initialized to 0

Current code uses kmalloc() and then just does a bitwise OR operation on
qp->flags in create_qp_common(), which means that qp->flags may
potentially have some unintended bits set.  This patch uses kzalloc()
and avoids further explicit clearing of structure members, which also
shrinks the code:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-65 (-65)
function                                     old     new   delta
create_qp_common                            2024    1959     -65

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>