If dev->dma_device->params == NULL then the maximum DMA segment size is 64
KB. See also the dma_get_max_seg_size() implementation. This patch fixes
the following kernel warning:

  DMA-API: infiniband rxe0: mapping sg segment longer than device claims to support [len=126976] [max=65536]
  WARNING: CPU: 4 PID: 4848 at kernel/dma/debug.c:1220 debug_dma_map_sg+0x3d9/0x450
  RIP: 0010:debug_dma_map_sg+0x3d9/0x450
  Call Trace:
   srp_queuecommand+0x626/0x18d0 [ib_srp]
   scsi_queue_rq+0xd02/0x13e0 [scsi_mod]
   __blk_mq_try_issue_directly+0x2b3/0x3f0
   blk_mq_request_issue_directly+0xac/0xf0
   blk_insert_cloned_request+0xdf/0x170
   dm_mq_queue_rq+0x43d/0x830 [dm_mod]
   __blk_mq_try_issue_directly+0x2b3/0x3f0
   blk_mq_request_issue_directly+0xac/0xf0
   blk_mq_try_issue_list_directly+0xb8/0x170
   blk_mq_sched_insert_requests+0x23c/0x3b0
   blk_mq_flush_plug_list+0x529/0x730
   blk_flush_plug_list+0x21f/0x260
   blk_mq_make_request+0x56b/0xf20
   generic_make_request+0x196/0x660
   submit_bio+0xae/0x290
   blkdev_direct_IO+0x822/0x900
   generic_file_direct_write+0x110/0x200
   __generic_file_write_iter+0x124/0x2a0
   blkdev_write_iter+0x168/0x270
   aio_write+0x1c4/0x310
   io_submit_one+0x971/0x1390
   __x64_sys_io_submit+0x12a/0x390
   do_syscall_64+0x6f/0x2e0
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

Link: https://lore.kernel.org/r/20191025225830.257535-2-bvanassche@acm.org
Cc: <stable@vger.kernel.org>
Fixes: 0b5cb330 ("RDMA/srp: Increase max_segment_size")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>