rtsock: fix socket closure.

rtsock: fix socket closure.

Currently `close(2)` erroneously return `EOPNOTSUPP` for `PF_ROUTE` sockets.
It happened after making rtsock socket implementation self-contained (
36b10ac2cd18 ). Rtsock code marks socket as connected in `rts_attach()`.
`soclose()` tries to disconnect such socket using `.pr_disconnect` callback.
Rtsock does not implement this callback, resulting in the default method being
substituted. This default method returns `ENOTSUPP`, failing `soclose()` logic.

This diff restores the previous behaviour by adding custom `pr_disconnect()`
returning `ENOTCONN`.

Reviewed by:	glebius
Differential Revision: https://reviews.freebsd.org/D38059