sk-inet: restore a value of SO_REUSEPORT

The SO_REUSEPORT option allows multiple sockets on the same
host to bind to the same port. This option has to ve restored when all
sockets are bound to a port. The same logic is already used to restore
SO_REUSEADDR.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>