link: fix memory leaks due to repeated calls to .io_alloc()

All the io_alloc() implementation unconditionally allocated
new memory, thus leaking memory if called more then once.
Fix io_alloc() implementations not to allocate new memory
if not needed.

This happens for example in link_msg_parser() which first
calls rtnl_link_set_type():

    #0  macvlan_alloc (link=0x609d50) at route/link/macvlan.c:56
    #1  0x00007ffff7b99a78 in rtnl_link_set_type (link=link@entry=0x609d50, type=type@entry=0x609a94 "macvlan") at route/link.c:2233
    #2  0x00007ffff7b99c28 in link_msg_parser (ops=<optimized out>, who=<optimized out>, n=<optimized out>, pp=0x7fffffffd870) at route/link.c:547
    #3  0x00007ffff7dea109 in nl_cache_parse (ops=0x7ffff7dd8600 <rtnl_link_ops>, who=0x603338, nlh=0x6098a0, params=0x7fffffffd870) at cache.c:914
    #4  0x00007ffff7dea15b in update_msg_parser (msg=<optimized out>, arg=<optimized out>) at cache.c:668
    #5  0x00007ffff7def7bf in nl_cb_call (msg=<optimized out>, type=<optimized out>, cb=<optimized out>) at ../include/netlink-private/netlink.h:142
    #6  recvmsgs (cb=0x6057a0, sk=0x6034c0) at nl.c:952
    #7  nl_recvmsgs_report (sk=sk@entry=0x6034c0, cb=cb@entry=0x6057a0) at nl.c:1003
    #8  0x00007ffff7defb79 in nl_recvmsgs (sk=sk@entry=0x6034c0, cb=cb@entry=0x6057a0) at nl.c:1027
    #9  0x00007ffff7de9668 in __cache_pickup (sk=0x6034c0, cache=0x603510, param=param@entry=0x7fffffffd870) at cache.c:701
    #10 0x00007ffff7dea08d in nl_cache_pickup (sk=<optimized out>, cache=<optimized out>) at cache.c:753
    #11 0x0000000000400d56 in main ()

and later ops->io_parse():

    #0  macvlan_alloc (link=0x609d50) at route/link/macvlan.c:56
    #1  0x00007ffff7baae9d in macvlan_parse (link=0x609d50, data=<optimized out>, xstats=<optimized out>) at route/link/macvlan.c:79
    #2  0x00007ffff7b99c80 in link_msg_parser (ops=<optimized out>, who=<optimized out>, n=<optimized out>, pp=0x7fffffffd870) at route/link.c:567
    #3  0x00007ffff7dea109 in nl_cache_parse (ops=0x7ffff7dd8600 <rtnl_link_ops>, who=0x603338, nlh=0x6098a0, params=0x7fffffffd870) at cache.c:914
    #4  0x00007ffff7dea15b in update_msg_parser (msg=<optimized out>, arg=<optimized out>) at cache.c:668
    #5  0x00007ffff7def7bf in nl_cb_call (msg=<optimized out>, type=<optimized out>, cb=<optimized out>) at ../include/netlink-private/netlink.h:142
    #6  recvmsgs (cb=0x6057a0, sk=0x6034c0) at nl.c:952
    #7  nl_recvmsgs_report (sk=sk@entry=0x6034c0, cb=cb@entry=0x6057a0) at nl.c:1003
    #8  0x00007ffff7defb79 in nl_recvmsgs (sk=sk@entry=0x6034c0, cb=cb@entry=0x6057a0) at nl.c:1027
    #9  0x00007ffff7de9668 in __cache_pickup (sk=0x6034c0, cache=0x603510, param=param@entry=0x7fffffffd870) at cache.c:701
    #10 0x00007ffff7dea08d in nl_cache_pickup (sk=<optimized out>, cache=<optimized out>) at cache.c:753
    #11 0x0000000000400d56 in main ()

https://github.com/thom311/libnl/issues/59

Signed-off-by: Thomas Haller <thaller@redhat.com>