bpf: dev_map_alloc() shouldn't return NULL

We forgot to set the error code on two error paths which means that we
return ERR_PTR(0) which is NULL.  The caller, find_and_alloc_map(), is
not expecting that and will have a NULL dereference.

Fixes: 546ac1ff ("bpf: add devmap, a map for storing net device references")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>