zebra: netlink_request consolidataion

zebra: netlink_request consolidataion

Make netlink_request api generic where it can be used
for dump or querying specific information request.

nelink request nlm flags (NLM_F_ROOT | NLM_F_MATCH) are
used to dump purpose, if client wants to query spcific
MAC or IP using netlink_request does not require to set
them.
nlm struct is passed by the caller of netlink_request,
it can also set the nlm request flags.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>