bgpd: Treat-as-withdraw attribute if remaining data is not enough

Relax this handling (RFC 7606) only for eBGP peers.

More details: https://datatracker.ietf.org/doc/html/rfc7606#section-4

There are two error cases in which the Total Attribute Length value
can be in conflict with the enclosed path attributes, which
themselves carry length values:

    * In the first case, the length of the last encountered path
    attribute would cause the Total Attribute Length to be exceeded
    when parsing the enclosed path attributes.

    * In the second case, fewer than three octets remain (or fewer than
    four octets, if the Attribute Flags field has the Extended Length
    bit set) when beginning to parse the attribute.  That is, this
    case exists if there remains unconsumed data in the path
    attributes but yet insufficient data to encode a single minimum-
    sized path attribute. <<<< HANDLING THIS CASE IN THIS COMMIT >>>>

In either of these cases, an error condition exists and the "treat-
as-withdraw" approach MUST be used (unless some other, more severe
error is encountered dictating a stronger approach), and the Total
Attribute Length MUST be relied upon to enable the beginning of the
NLRI field to be located.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

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