net: bridge: reduce indentation level in fdb_create

We can express the same logic without an "if" condition as big as the
function, just return early if the kmem_cache_alloc() call fails.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>