coredump: validate socket name as it is written

In contrast to other parameters written into
/proc/sys/kernel/core_pattern that never fail we can validate enabling
the new AF_UNIX support. This is obviously racy as hell but it's always
been that way.

Link: https://lore.kernel.org/20250516-work-coredump-socket-v8-7-664f3caf2516@kernel.org


Acked-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>