net: cache skb_shinfo() in skb_try_coalesce()

Compiler does not really know that skb_shinfo(to|from) are constants
in skb_try_coalesce(), lets cache their values to shrink code.

We might even take care of skb_zcopy() calls later.

$ size net/core/skbuff.o.before net/core/skbuff.o
   text	   data	    bss	    dec	    hex	filename
  40727	   1298	      0	  42025	   a429	net/core/skbuff.o.before
  40631	   1298	      0	  41929	   a3c9	net/core/skbuff.o

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