vnet: Ensure the space allocated by vnet_data_alloc() is sufficent aligned

Some 32-bit architectures, e.g., armv7, require strict 8-byte
alignment while doing atomic 64-bit access. Hence aligning to the
pointer type (4-byte alignment) does not meet the requirement on
those architectures.

Make the space allocated by vnet_data_alloc() sufficent aligned to
avoid unaligned access.

PR: 265639
Diagnosed by: markj
Reviewed by: jhb, markj
Co-authored-by: jhb
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D55560