nbd/server: Ignore write errors when replying to NBD_OPT_ABORT

nbd/server: Ignore write errors when replying to NBD_OPT_ABORT

Commit 37ec36f6 intentionally ignores errors when trying to reply
to an NBD_OPT_ABORT request for plaintext clients, but did not make
the same change for a TLS server.  Since NBD_OPT_ABORT is
documented as being a potential for an EPIPE when the client hangs
up without waiting for our reply, we don't need to pollute the
server's output with that failure.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181117223221.2198751-1-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>