dump_attrs: "NLA_F_NESTED"  => nla_is_nested(nla)
Fix leak of cb if nl_socket_alloc_cb() failed to allocate socket

- each *_get() should have corresponding *_put(). That rule was broken in nl_socket_alloc()
- Also, check if cb is NULL in nl_socket_set_cb (calls BUG())
attr: nla_is_nested() must access nla_type directly

Can't used nla_type() as it applies NLA_TYPE_MASK first

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Merge branch 'fixes' of https://github.com/socketpair/libnl