fou: remove warn in gue_gro_receive on unsupported protocol
In the Linux kernel, the following vulnerability has been resolved:

fou: remove warn in gue_gro_receive on unsupported protocol

Drop the WARN_ON_ONCE inn gue_gro_receive if the encapsulated type is
not known or does not have a GRO handler.

Such a packet is easily constructed. Syzbot generates them and sets
off this warning.

Remove the warning as it is expected and not actionable.

The warning was previously reduced from WARN_ON to WARN_ON_ONCE in
commit 270136613bf7 ("fou: Do WARN_ON_ONCE in gue_gro_receive for bad
proto callbacks").