netfilter: ctnetlink: fix return value of ctnetlink_get_expect()

This fixes one bogus error that is returned to user-space:

libnetfilter_conntrack/utils# ./expect_get
TEST: get expectation (-1)(Unknown error 18446744073709551504)

This patch includes the correct handling for EAGAIN (nfnetlink
uses this error value to restart the operation after module
auto-loading).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>