xfrm: remove the fragment check for ipv6 beet mode

In commit 68dc022d ("xfrm: BEET mode doesn't support fragments
for inner packets"), it tried to fix the issue that in TX side the
packet is fragmented before the ESP encapping while in the RX side
the fragments always get reassembled before decapping with ESP.

This is not true for IPv6. IPv6 is different, and it's using exthdr
to save fragment info, as well as the ESP info. Exthdrs are added
in TX and processed in RX both in order. So in the above case, the
ESP decapping will be done earlier than the fragment reassembling
in TX side.

Here just remove the fragment check for the IPv6 inner packets to
recover the fragments support for BEET mode.

Fixes: 68dc022d ("xfrm: BEET mode doesn't support fragments for inner packets")
Reported-by: Xiumei Mu <xmu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>