RDMA/rxe: Fix configuration of atomic queue pair attributes

RDMA/rxe: Fix configuration of atomic queue pair attributes

From the comment above the definition of the roundup_pow_of_two() macro:

     The result is undefined when n == 0.

Hence only pass positive values to roundup_pow_of_two(). This patch fixes
the following UBSAN complaint:

  UBSAN: Undefined behaviour in ./include/linux/log2.h:57:13
  shift exponent 64 is too large for 64-bit type 'long unsigned int'
  Call Trace:
   dump_stack+0xa5/0xe6
   ubsan_epilogue+0x9/0x26
   __ubsan_handle_shift_out_of_bounds.cold+0x4c/0xf9
   rxe_qp_from_attr.cold+0x37/0x5d [rdma_rxe]
   rxe_modify_qp+0x59/0x70 [rdma_rxe]
   _ib_modify_qp+0x5aa/0x7c0 [ib_core]
   ib_modify_qp+0x3b/0x50 [ib_core]
   cma_modify_qp_rtr+0x234/0x260 [rdma_cm]
   __rdma_accept+0x1a7/0x650 [rdma_cm]
   nvmet_rdma_cm_handler+0x1286/0x14cd [nvmet_rdma]
   cma_cm_event_handler+0x6b/0x330 [rdma_cm]
   cma_ib_req_handler+0xe60/0x22d0 [rdma_cm]
   cm_process_work+0x30/0x140 [ib_cm]
   cm_req_handler+0x11f4/0x1cd0 [ib_cm]
   cm_work_handler+0xb8/0x344e [ib_cm]
   process_one_work+0x569/0xb60
   worker_thread+0x7a/0x5d0
   kthread+0x1e6/0x210
   ret_from_fork+0x24/0x30

Link: https://lore.kernel.org/r/20200217205714.26937-1-bvanassche@acm.org
Fixes: 8700e3e7c485 ("Soft RoCE driver")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>