net: gro: dev_gro_receive() cleanup

__napi_gro_receive() is inlined from two call sites for no good reason.

Lets move the prep stuff in a function of its own, called only if/when
needed. This saves 300 bytes on x86 :

# size net/core/dev.o.after net/core/dev.o.before
   text	   data	    bss	    dec	    hex	filename
  51968	   1238	   1040	  54246	   d3e6	net/core/dev.o.before
  51664	   1238	   1040	  53942	   d2b6	net/core/dev.o.after

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>