Permit disabling net.inet.udp.require_l2_bcast in VIMAGE kernels.

The default value of the tunable introduced in r304436 couldn't be
effectively overrided on VIMAGE kernels, because instead of being
accessed via the appropriate VNET() accessor macro, it was accessed
via the VNET_NAME() macro, which resolves to the (should-be) read-only
master template of initial values of per-VNET data. Hence, while the
value of udp_require_l2_bcast could be altered on per-VNET basis, the
code in udp_input() would ignore it as it would always read the default
value (one) from the VNET master template.

Silence from: rstone