srp_daemon: run port services without PrivateNetwork

Since kernel v7.3, commit e9153a267119 ("RDMA/srp: Make the SRP sysfs
class net namespace aware") tags the infiniband_srp sysfs class with
the net namespace of the underlying RDMA device. The
srp_daemon_port@.service unit runs srp_daemon with PrivateNetwork=yes,
so the daemon now sees an empty /sys/class/infiniband_srp/ inside its
private network namespace and fails to log in to any target, endlessly
logging:

  srp_daemon: unable to open /sys/class/infiniband_srp/srp-mlx5_0-1/add_target, maybe ib_srp is not loaded

even though ib_srp is loaded and writing add_target from a root shell
in the init netns works. This makes the SRP initiator unusable through
the standard systemd service on any v7.3+ kernel.

srp_daemon fundamentally needs to run in the net namespace of the RDMA
device it manages, both for the umad device and now for the sysfs
class, so network namespace isolation is not applicable to this
service. Set PrivateNetwork=no.

Verified on Ubuntu 26.10 (kernel 7.3.0-9-generic) against an SCST
target: with PrivateNetwork=yes no login ever happens; with this change
both ports log in within seconds.

Signed-off-by: Aurelien Aptel <aaptel@nvidia.com>