AF_UNIX: Hold a reference of the unp_conn before executing blocking code
Since unp_token will be automaticly released upon execution of blocking
code, close of unp_conn could race any code paths that references unp_conn
after executing blocking code. To fix these races, we simply increment
the reference count of the unp_conn before executig any possibly blocking
code and release the reference count of unp_conn, which may release unp_conn
itself. This _currently_ does not suffer 0-ref race, since unp_token is
always being held.