selftests: fib_nexthop_multiprefix: fix cleanup() netns deletion

selftests: fib_nexthop_multiprefix: fix cleanup() netns deletion

During setup():
...
        for ns in h0 r1 h1 h2 h3
        do
                create_ns ${ns}
        done
...

while in cleanup():
...
        for n in h1 r1 h2 h3 h4
        do
                ip netns del ${n} 2>/dev/null
        done
...

and after removing the stderr redirection in cleanup():

$ sudo ./fib_nexthop_multiprefix.sh
...
TEST: IPv4: host 0 to host 3, mtu 1400                              [ OK ]
TEST: IPv6: host 0 to host 3, mtu 1400                              [ OK ]
Cannot remove namespace file "/run/netns/h4": No such file or directory
$ echo $?
1

and a non-zero return code, make kselftests fail (even if the test
itself is fine):

...
not ok 34 selftests: net: fib_nexthop_multiprefix.sh # exit=1
...

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>