staging/lustre/lnet: Fix potential uninitialized variable warning

Greg reports that with one of the patches in his queue
there's now an unused variable warning in lnet_parse_ip2nets
for ipaddrs variable.
Apparently the warning is a false positive as in all cases where
lnet_ipaddr_enumerate can return without setting ipaddrs to something
a negative return value is returned that is then checked
before we actually use ipaddrs.

Assign ipaddrs to NULL to quiet this.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>