net: Add getsockopt support for TCP thin-streams

Initial TCP thin-stream commit did not add getsockopt support for the new
socket options: TCP_THIN_LINEAR_TIMEOUTS and TCP_THIN_DUPACK. This adds support
for them.

Signed-off-by: Josh Hunt <johunt@akamai.com>
Tested-by: Andreas Petlund <apetlund@simula.no>
Acked-by: Andreas Petlund <apetlund@simula.no>
Signed-off-by: David S. Miller <davem@davemloft.net>