selftests: fib_tests: Disable RP filter in multipath list receive test

The test relies on the fib:fib_table_lookup trace point being triggered
once for each forwarded packet. If RP filter is not disabled, the trace
point will be triggered twice for each packet (for source validation and
forwarding), potentially masking actual bugs. Fix by explicitly
disabling RP filter.

Before:

 # ./fib_tests.sh -t ipv4_mpath_list

 IPv4 multipath list receive tests
     TEST: Multipath route hit ratio (1.99)                              [ OK ]

After:

 # ./fib_tests.sh -t ipv4_mpath_list

 IPv4 multipath list receive tests
     TEST: Multipath route hit ratio (.99)                               [ OK ]

Fixes: 8ae9efb8 ("selftests: fib_tests: Add multipath list receive tests")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/netdev/202309191658.c00d8b8-oliver.sang@intel.com/


Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Tested-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
Link: https://lore.kernel.org/r/20231010132113.3014691-2-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>