[IPX]: Annotate and fix IPX checksum

Calculation of IPX checksum got buggered about 2.4.0.  The old variant
mangled the packet; that got fixed, but calculation itself got buggered.
Restored the correct logics, fixed a subtle breakage we used to have even
back then: if the sum is 0 mod 0xffff, we want to return 0, not 0xffff.
The latter has special meaning for IPX (cheksum disabled).  Observation
(and obvious fix) nicked from history of FreeBSD ipx_cksum.c...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 2 deletions

diff --git a/include/net/ipx.h b/include/net/ipx.h
index 4a423d2695ba..c6b2ee610866 100644
--- a/include/net/ipx.h
+++ b/include/net/ipx.h
@@ -26,8 +26,8 @@ struct ipx_address {
 #define IPX_MAX_PPROP_HOPS 8
 
 struct ipxhdr {
-        __u16                   ipx_checksum __attribute__ ((packed));
-#define IPX_NO_CHECKSUM 0xFFFF
+        __be16                  ipx_checksum __attribute__ ((packed));
+#define IPX_NO_CHECKSUM __constant_htons(0xFFFF)
         __be16                  ipx_pktsize __attribute__ ((packed));
         __u8                    ipx_tctrl;
         __u8                    ipx_type;