nfp: bpf: make array exp_mask static, makes object smaller

Don't populate the array exp_mask on the stack but instead make it
static. Makes the object code smaller by 224 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  77832	   2290	      0	  80122	  138fa	ethernet/netronome/nfp/bpf/jit.o

After:
   text	   data	    bss	    dec	    hex	filename
  77544	   2354	      0	  79898	  1381a	ethernet/netronome/nfp/bpf/jit.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>