This is trivial patch for libnetlink.c in iproute2.

In iproute2/include/linux/netlink.h NLM_F_DUMP is defines as:

   #define NLM_F_DUMP	(NLM_F_ROOT|NLM_F_MATCH)

It is not used in libnetlink.c. If used, the code becomes a bit easier
to read.
Signed-off-by: Masatake YAMATO <yamato@redhat.com>