bpf, xskmap: fix crash in xsk_map_alloc error path handling

If bpf_map_precharge_memlock() did not fail, then we set err to zero.
However, any subsequent failure from either alloc_percpu() or the
bpf_map_area_alloc() will return ERR_PTR(0) which in find_and_alloc_map()
will cause NULL pointer deref.

In devmap we have the convention that we return -EINVAL on page count
overflow, so keep the same logic here and just set err to -ENOMEM
after successful bpf_map_precharge_memlock().

Fixes: fbfc504a ("bpf: introduce new bpf AF_XDP map type BPF_MAP_TYPE_XSKMAP")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Björn Töpel <bjorn.topel@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/xskmap.c

@@ -56,6 +56,8 @@ static struct bpf_map *xsk_map_alloc(union bpf_attr *attr)
 	if (err)
 		goto free_m;
 
+	err = -ENOMEM;
+
 	m->flush_list = alloc_percpu(struct list_head);
 	if (!m->flush_list)
 		goto free_m;