IB/mlx5: Replace kfree with kvfree

Memory allocated by kvzalloc should not be freed by kfree(), use kvfree()
instead.

Fixes: 813e90b1

 ("IB/mlx5: Add advise_mr() support")
Link: https://lore.kernel.org/r/20190717082101.14196-1-hslester96@gmail.com
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>