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: 1fa3314c ("ipv4: Centralize TOS matching")
Reported-by: David Ahern <dsahern@kernel.org>
Closes: https://lore.kernel.org/netdev/2f5146ff-507d-4cab-a195-b28c0c9e654e@kernel.org/Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20240903133554.2807343-1-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>