KVM: Fix the bitmap range to copy during clear dirty

mainline inclusion
from mainline-v5.10
commit: 4ddc9204
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I66COX
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.10.y&id=4ddc9204572c33f2eb91fbdb1d99d8078388b67d



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kvm_dirty_bitmap_bytes() will return the size of the dirty bitmap of
the memslot rather than the size of bitmap passed over from the ioctl.
Here for KVM_CLEAR_DIRTY_LOG we should only copy exactly the size of
bitmap that covers kvm_clear_dirty_log.num_pages.

Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 2a31b9db
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>