bpf: sockmap, fix double-free

`e' is being freed twice.

Fix this by removing one of the kfree() calls.

Addresses-Coverity-ID: 1468983 ("Double free")
Fixes: 81110384

 ("bpf: sockmap, add hash map support")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>