vxlan: validate ND option lengths in vxlan_na_create

In the Linux kernel, the following vulnerability has been resolved:

vxlan: validate ND option lengths in vxlan_na_create

vxlan_na_create() walks ND options according to option-provided

lengths. A malformed option can make the parser advance beyond the

computed option span or use a too-short source LLADDR option payload.

Validate option lengths against the remaining NS option area before

advancing, and only read source LLADDR when the option is large enough

for an Ethernet address.