user_ns: Prepare creds of newly created task

Sockets are sent via SCM_CREDENTIALS, and this kernel interface
needs to have uid and gid mapped (see __scm_send() in kernel).
So, set them before send_fds() use.

Also, move prep_usernsd_transport() below to be after this
for uniformity.

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