The function at issue does not fully validate the content of the
structure pointed by the log parameter, though its content has just been
copied from userspace and lacks validation. Fix that.

Moreover, change the type of n to unsigned long as that is the type
returned by kvm_dirty_bitmap_bytes().

Signed-off-by: Tomas Bortoli <tomasbortoli@gmail.com>
Reported-by:  <syzbot+028366e52c9ace67deb3@syzkaller.appspotmail.com>
[Squashed the fix from Paolo. - Radim.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>