dccp: make const array error_code static

Don't populate array error_code on the stack but make it static. Makes
the object code smaller by almost 250 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  10366	    983	      0	  11349	   2c55	net/dccp/input.o

After:
   text	   data	    bss	    dec	    hex	filename
  10161	   1039	      0	  11200	   2bc0	net/dccp/input.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>