Phonet: remove dangling pipe if an endpoint is closed early

Closing a pipe endpoint is not normally allowed by the Phonet pipe,
other than as a side after-effect of removing the pipe between two
endpoints. But there is no way to prevent Linux userspace processes
from being killed or suffering from bugs, so this can still happen.
We might as well forcefully close Phonet pipe endpoints then.

The cellular modem supports only a few existing pipes at a time. So we
really should not leak them. This change instructs the modem to destroy
the pipe if either of the pipe's endpoint (Linux socket) is closed too
early.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>