kernel linker: Eliminate unneeded vnet propagation

The module preload happens before vnet0 creation, at this moment the vnet
list is empty thus invoking vnet_data_copy() during preload is a noop.

With recent change 110113bc086f, for dynamic module load, aka via kldload,
linker will do vnet propagation right after registering sysctls which
happens after module load, then previous propagation (during module load)
is redundant.

No functional change intended.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D39852