net: ieee802154: forbid monitor for del llsec seclevel

commit 9dde130937e95b72adfae64ab21d6e7e707e2dac upstream.

This patch forbids to del llsec seclevel for monitor interfaces which we
don't support yet. Otherwise we will access llsec mib which isn't
initialized for monitors.

Reported-by:  <syzbot+fbf4fc11a819824e027b@syzkaller.appspotmail.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210405003054.256017-15-aahringo@redhat.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>