shared/netutils/network/linux/cgo: Don't call net.InterfaceByIndex in NetnsGetifaddrs
shared/netutils/network/linux/cgo: Don't call net.InterfaceByIndex in NetnsGetifaddrs

As calling it repeatedly for a list of instance NICs is inefficient as internally it loads
info about all interfaces.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd