RDMA/rxe: Zero out index member of struct rxe_queue

1) New index member of struct rxe_queue was introduced but not zeroed so
   the initial value of index may be random.

2) The current index is not masked off to index_mask.

In this case producer_addr() and consumer_addr() will get an invalid
address by the random index and then accessing the invalid address
triggers the following panic:

"BUG: unable to handle page fault for address: ffff9ae2c07a1414"

Fix the issue by using kzalloc() to zero out index member.

Fixes: 5bcf5a59

 ("RDMA/rxe: Protext kernel index from user space")
Link: https://lore.kernel.org/r/20210820111509.172500-1-yangx.jy@fujitsu.com
Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>