net/mlx5: Allocate enough space for the FDB sub-namespaces

FDB_MAX_CHAIN is three.  We wanted to allocate enough memory to hold four
structs but there are missing parentheses so we only allocate enough
memory for three structs and the first byte of the fourth one.

Fixes: 328edb49 ("net/mlx5: Split FDB fast path prio to multiple namespaces")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>