libceph: get rid of more_kvec in try_write()

All gotos to "more" are conditioned on con->state == OPEN, but the only
thing "more" does is opening the socket if con->state == PREOPEN.  Kill
that label and rename "more_kvec" to "more".

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>