gro: remove a sparse error

Fix following sparse error :

net/ipv4/af_inet.c:1410:59: warning: restricted __be16 degrades to
integer

added in commit db8caf3d
("gro: should aggregate frames without DF")

Reported-by: kbuild test robot <fengguang.wu@intel.com>
From: Eric Dumazet <edumazet@google.com>

Signed-off-by: David S. Miller <davem@davemloft.net>