vxlan: Update vxlan fdb 'used' field after each usage

Fix some instances where vxlan fdb 'used' field is not updated after the entry
is used.

v2: rename vxlan_find_mac() as __vxlan_find_mac() and create a new vxlan_find_mac()
that also updates ->used field.

Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>