Avoid VLA[1] by using an already allocated buffer passed
by the caller.

[1] https://lkml.org/lkml/2018/3/7/621Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>