ipv6: remove min MTU check for ipsec tunnels

With 749439bfac ipsec dst's that report a MTU less than
IPV6_MIN_MTU are broken even for packets that are smaller
than IPV6_MIN_MTU.

According to rfc2473#section-7.1

    if the original IPv6 packet is equal or  smaller  than  the
    IPv6 minimum link MTU, the tunnel entry-point node
    encapsulates the original packet, and subsequently
    fragments the resulting IPv6 tunnel packet into IPv6
    fragments that do not exceed the Path MTU to the tunnel
    exit-point.

This patch drops the MTU check for ipsec tunnel destinations.

Change-Id: I36152ec99a955e20ded707fd7269ba11b94a9cfc
Signed-off-by: Ashwanth Goli <ashwanth@codeaurora.org>
Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>