net: hsr: reject unresolved interlink ifindex

net: hsr: reject unresolved interlink ifindex

In hsr_newlink(), a provided but invalid IFLA_HSR_INTERLINK attribute
was silently ignored if __dev_get_by_index() returned NULL. This leads
to incorrect RedBox topology creation without notifying the user.

Fix this by returning -EINVAL and an extack message when the
interlink attribute is present but cannot be resolved.

Reviewed-by: Felix Maurer <fmaurer@redhat.com>
Signed-off-by: Luka Gejak <luka.gejak@linux.dev>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20260513182657.20346-3-luka.gejak@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>