Improve efficiency of lxc_ifname_alnum_case_sensitive

Improve efficiency of lxc_ifname_alnum_case_sensitive

To detect if a newly generated interface name is a duplicate of an existing interface lxc_ifname_alnum_case_sensitive() currently gets a list of all interfaces using netns_getifaddrs(). When the system has a small number of interfaces this works fine, however when there are thousands or tens of thousands of interfaces this quickly becomes less than optimal.

As we only need to check if an interface name exists, and do not need the detailed information about the interfaces provided by netns_getifaddrs(), we can instead use the if_nametoindex() function, which is much more efficient.

Signed-off-by: Sam Boyles <sam.boyles@alliedtelesis.co.nz>