Each struct vm_mem_backing_src_alias has a flags field, which denotes
the flags used to mmap() an area of that type. Previously, this field
never included MAP_PRIVATE | MAP_ANONYMOUS, because
vm_userspace_mem_region_add assumed that *all* types would always use
those flags, and so it hardcoded them.

In a follow-up commit, we'll add a new type: shmem. Areas of this type
must not have MAP_PRIVATE | MAP_ANONYMOUS, and instead they must have
MAP_SHARED.

So, refactor things. Make it so that the flags field of
struct vm_mem_backing_src_alias really is a complete set of flags, and
don't add in any extras in vm_userspace_mem_region_add. This will let us
easily tack on shmem.
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Message-Id: <20210519200339.829146-7-axelrasmussen@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>