Manually padding and aligning the mmap region is only needed when using
THP. When using HugeTLB, mmap will always return an address aligned to
the HugeTLB page size. Add a comment to clarify this and assert the mmap
behavior for HugeTLB.

[Removed requirement that HugeTLB mmaps must be padded per Yanan's
 feedback and added assertion that mmap returns aligned addresses
 when using HugeTLB.]

Cc: Ben Gardon <bgardon@google.com>
Cc: Yanan Wang <wangyanan55@huawei.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Aaron Lewis <aaronlewis@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20211111000310.1435032-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>