mlxsw: spectrum_switchdev: Check status of memory allocation

We can't rely on kzalloc() always succeeding, so check its return value.

Suppresses the following smatch error:

mlxsw_sp_switchdev_event() error: potential null dereference
'switchdev_work->fdb_info.addr'.  (kzalloc returns
 null)

Fixes: af061378 ("mlxsw: spectrum_switchdev: Add support for learning FDB through notification")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>