rdma/sys.c: fix possible out-of-bound array access

rdma/sys.c: fix possible out-of-bound array access

netns_modes_str[] array has 2 elements, when netns_mode is 2,
condition (2 <= 2) will be true and `mode_str = netns_modes_str[2]'
will be executed, which will result in out-of-bound read.

Signed-off-by: Michał Łyszczek <michal.lyszczek@bofc.pl>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>