phonet/pep: refuse to enable an unbound pipe

This ioctl() implicitly assumed that the socket was already bound to
a valid local socket name, i.e. Phonet object. If the socket was not
bound, two separate problems would occur:

1) We'd send an pipe enablement request with an invalid source object.
2) Later socket calls could BUG on the socket unexpectedly being
   connected yet not bound to a valid object.

Reported-by:  <syzbot+2dc91e7fc3dea88b1e8a@syzkaller.appspotmail.com>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: David S. Miller <davem@davemloft.net>