mlx4_ib: zero out struct ib_pd when allocating

Zero out the fields of the struct ib_pd for user mode pds so that
users querying pds via nldev will not get garbage.  For simplicity,
use kzalloc() to allocate the mlx4_ib_pd struct.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>