arm64: Fix the VM_MAX_NAMELEN definition

The maximum VM name length has changed since the arm64 vmm code was
forked. For now, just sync with the amd64 definitions, since they
permit longer VM names. As arm64/vmm is implemented only in main, I
won't bother dealing with backward compat.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D46484