Improve batch and dump times by caching link lookups

Improve batch and dump times by caching link lookups

ip route uses ll_name_to_index and ll_index_to_name to convert between
device names and indices. At the moment both use for the ioctl based glibc
functions if_nametoindex and if_indextoname and does not cache the result.
When using a batch file or dumping large number of routes this means the
same device lookups can be done repeatedly adding unnecessary overhead
(socket + ioctl + close for each device lookup).

Add a new function, ll_link_get, to send a netlink based RTM_GETLINK. If
successful, cache the result in idx_head and name_head so future lookups
can re-use the entry. Update ll_name_to_index and ll_index_to_name to use
ll_link_get and only fallback to the glibc functions if it fails.

With this change the time to install 720,022 routes with 2 ecmp nexthops
where the nexthop device is given is reduced from 31.4 seconds to 19.2
seconds. A dump of those routes drops from 13.3 to 2.8 seconds.

Signed-off-by: David Ahern <dsahern@gmail.com>