selftests: mptcp: add TCP_INQ support

Do checks on the returned inq counter.

Fail on:
1. Huge value (> 1 kbyte, test case files are 1 kb)
2. last hint larger than returned bytes when read was short
3. erronenous indication of EOF.

3) happens when a hint of X bytes reads X-1 on next call
   but next recvmsg returns more data (instead of EOF).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>