vxlan: don't age NTF_EXT_LEARNED fdb entries

vxlan driver already implicitly supports installing
of external fdb entries with NTF_EXT_LEARNED. This
patch just makes sure these entries are not aged
by the vxlan driver. An external entity managing these
entries will age them out. This is consistent with
the use of NTF_EXT_LEARNED in the bridge driver.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>