selftests/net: convert fib_nexthop_multiprefix to run it in unique namespace

Here is the test result after conversion.

 ]# ./fib_nexthop_multiprefix.sh
 TEST: IPv4: host 0 to host 1, mtu 1300                              [ OK ]
 TEST: IPv6: host 0 to host 1, mtu 1300                              [ OK ]

 TEST: IPv4: host 0 to host 2, mtu 1350                              [ OK ]
 TEST: IPv6: host 0 to host 2, mtu 1350                              [ OK ]

 TEST: IPv4: host 0 to host 3, mtu 1400                              [ OK ]
 TEST: IPv6: host 0 to host 3, mtu 1400                              [ OK ]

 TEST: IPv4: host 0 to host 1, mtu 1300                              [ OK ]
 TEST: IPv6: host 0 to host 1, mtu 1300                              [ OK ]

 TEST: IPv4: host 0 to host 2, mtu 1350                              [ OK ]
 TEST: IPv6: host 0 to host 2, mtu 1350                              [ OK ]

 TEST: IPv4: host 0 to host 3, mtu 1400                              [ OK ]
 TEST: IPv6: host 0 to host 3, mtu 1400                              [ OK ]

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-8-liuhangbin@gmail.com


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