net: ipv6: RA prefix option length not checked

The Router Advertisement can have prefix option. It's length
is 4 but the code did not check that which meant that we could
accept malformed packet. See RFC 4861 chapter 4.6.2 for details.

Fixes #25694

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>