netfilter: xt_qtaguid: Allow tracking loopback

 In the past it would always ignore interfaces with loopback addresses.
 Now we just treat them like any other.
 This also helps with writing tests that check for the presence
 of the qtaguid module.

 Inspired-by: JP Abgrall <jpa@google.com>
 Patchset available at
 https://github.com/boundarydevices/linux-imx6/commit/ec99b775eb78b58a23db9af6009b02b3ad4abf7b
 netfilter: qtaguid: rate limit some of the printks

 Some of the printks are in the packet handling path.
 We now ratelimit the very unlikely errors to avoid
 kmsg spamming.

 Original-work-by: JP Abgrall <jpa@google.com>
 https://github.com/boundarydevices/linux-imx6/commit/3b5720b87f77592ce784492a65ff448f7cc4dd19