gre: Workarounds for large packets over GRE

gre: Workarounds for large packets over GRE

The first change is to not propagate the IP DF bit from the inner
packet to the outer packet.  Large TCP packets can get segmented
first which will set the DF bit.  However these segmented packets
might still be too large after the GRE header is added, requiring
fragmentation.

The second change is to raise the MTU of the GRE tunnel device.
This prevents packets from being dropped in the datapath before
they can be fragmented.  Since the datapath is layer 2 it does not
do any fragmentation and drops any packets that are too large.

Both of these are temporary workarounds that need to be addressed
more carefully in the future.

Bug #2379