IB/rxe: replace kvfree with vfree

The buf is allocated by vmalloc_user in the function rxe_queue_init.
So it is better to free it by vfree.

Fixes: 8700e3e7

 ("Soft RoCE driver")
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>