pf: use sctp_calculate_cksum()

[kp]

Summary

This function is always available, even if the SCTP or SCTP_SUPPORT options
are not set.
That lets us remove an ifdef, and also means we improve pf's SCTP handling
when the options are not set.

MFC after: 3 weeks
Sponsored by: Orange Business Services

sys/netpfil/pf/pf.c

for (color = 0; color < 16; ++color) {
 
                         for (j = 0; j < 8; ++j) {
                                 if (!((mask >> (7 - j)) & 0x1))
                                         continue;
 
                                 /* The pixel "j" uses color "color". */
                                 for (plane = 0; plane < 4; ++plane)
                                         planes[i * 4 + plane] |=
-                                            ((color >> plane) & 0x1) << (7 - j);
+                                            ((cons_to_vga_colors[color] >>
+                                            plane) & 0x1) << (7 - j);
                         }
                 }
         }
 
         /*
          * The bytes are ready: we now switch to Write Mode 0 and write
          * all bytes, one plane at a time.
          */