net: ipv6: router advertisement message length should be within limits

The argument len passed to function process_ra is the length of the IPv6
router advertisement message and needs to be between 0 and MTU because
it is assigned to remaining_option_len and used as a loop variable.

Addresses-Coverity-ID: 450971 ("TAINTED_SCALAR")
Signed-off-by: Ehsan Mohandesi <emohandesi@linux.microsoft.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>