IPoIB: use kvzalloc to allocate an array of bucket pointers

This table by default takes 32KiB which is 3rd memory order. Meanwhile,
this memory is not aimed for DMA operation and could be safely allocated
by vmalloc.

Signed-off-by: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>