netconsole: restore userdatum value on update_userdata() failure

netconsole: restore userdatum value on update_userdata() failure

userdatum_value_store() updates udm->value first and only then calls
update_userdata() to rebuild the on-the-wire payload. If
update_userdata() fails (e.g. -ENOMEM from kmalloc), the function
returns the error to userspace, but udm->value already holds the new
string while the live nt->userdata buffer still reflects the old one.

The next successful write to any sibling userdatum on the same target
will call update_userdata() again, which walks every entry and packs
the now-stale udm->value into the payload. The failed write is thus
silently activated later, with no indication to userspace that the
value it tried to set was rejected.

Snapshot the previous value before overwriting udm->value and restore
it if update_userdata() fails so the visible state and the active
payload stay consistent.

Fixes: eb83801af2dc ("netconsole: Dynamic allocation of userdata buffer")
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260427-netconsole_ai_fixes-v2-4-59965f29d9cc@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>