net/mlx5e: Fix nullptr in mlx5e_tc_add_fdb_flow()

The result of __dev_get_by_index() is not checked for NULL, which then
passed to mlx5e_attach_encap() and gets dereferenced.

Also, in case of a successful lookup, the net_device reference count is
not incremented, which may result in net_device pointer becoming invalid
at any time during mlx5e_attach_encap() execution.

Fix by using dev_get_by_index(), which does proper reference counting on
the net_device pointer. Also, handle nullptr return value when mirred
device is not found.

It's safe to call dev_put() on the mirred net_device pointer, right
after mlx5e_attach_encap() call, because it's not being saved/copied
down the call chain.

Fixes: 3c37745e ("net/mlx5e: Properly deal with encap flows add/del under neigh update")
Addresses-Coverity: ("Dereference null return value")
Signed-off-by: Dima Chumak <dchumak@nvidia.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>