ulog: avoid accidental /dev/kmsg creation

Race is possible in ulog_kmsg(): if no /dev/kmsg exists
(e.g. while /dev gets re-mounted) regular file created instead.
>From this point system goes without kernel logger:
special character file can't be created anymore, all clients keep
overwriting single message in regular file.

To avoid this we open file in "r+" mode which doesn't create
file if it's not found.

Signed-off-by: Sergiy Kibrik <sakib@meta.ua>
Cc: John Crispin <blogic@openwrt.org>