ipv4: Fix user space build failure due to header change

RT_TOS() from include/uapi/linux/in_route.h is defined using
IPTOS_TOS_MASK from include/uapi/linux/ip.h. This is problematic for
files such as include/net/ip_fib.h that want to use RT_TOS() as without
including both header files kernel compilation fails:

In file included from ./include/net/ip_fib.h:25,
                 from ./include/net/route.h:27,
                 from ./include/net/lwtunnel.h:9,
                 from net/core/dst.c:24:
./include/net/ip_fib.h: In function ‘fib_dscp_masked_match’:
./include/uapi/linux/in_route.h:31:32: error: ‘IPTOS_TOS_MASK’ undeclared (first use in this function)
   31 | #define RT_TOS(tos)     ((tos)&IPTOS_TOS_MASK)
      |                                ^~~~~~~~~~~~~~
./include/net/ip_fib.h:440:45: note: in expansion of macro ‘RT_TOS’
  440 |         return dscp == inet_dsfield_to_dscp(RT_TOS(fl4->flowi4_tos));

Therefore, cited commit changed linux/in_route.h to include linux/ip.h.
However, as reported by David, this breaks iproute2 compilation due
overlapping definitions between linux/ip.h and
/usr/include/netinet/ip.h:

In file included from ../include/uapi/linux/in_route.h:5,
                 from iproute.c:19:
../include/uapi/linux/ip.h:25:9: warning: "IPTOS_TOS" redefined
   25 | #define IPTOS_TOS(tos)          ((tos)&IPTOS_TOS_MASK)
      |         ^~~~~~~~~
In file included from iproute.c:17:
/usr/include/netinet/ip.h:222:9: note: this is the location of the previous definition
  222 | #define IPTOS_TOS(tos)          ((tos) & IPTOS_TOS_MASK)

Fix by changing include/net/ip_fib.h to include linux/ip.h. Note that
usage of RT_TOS() should not spread further in the kernel due to recent
work in this area.

Fixes: 1fa3314 ("ipv4: Centralize TOS matching")
Reported-by: David Ahern <[email protected]>
Closes: https://lore.kernel.org/netdev/[email protected]/
Signed-off-by: Ido Schimmel <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Reviewed-by: Guillaume Nault <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

include/net/ip_fib.h

@@ -22,6 +22,7 @@
 #include <linux/percpu.h>
 #include <linux/notifier.h>
 #include <linux/refcount.h>
+#include <linux/ip.h>
 #include <linux/in_route.h>
 
 struct fib_config {

include/uapi/linux/in_route.h

@@ -2,8 +2,6 @@
 #ifndef _LINUX_IN_ROUTE_H
 #define _LINUX_IN_ROUTE_H
 
-#include <linux/ip.h>
-
 /* IPv4 routing cache flags */
 
 #define RTCF_DEAD	RTNH_F_DEAD