Three fixes all related to #DB:

 - Handle the BTF bit correctly so it doesn't get lost due to a kernel #DB

 - Only clear and set the virtual DR6 value used by ptrace on user space
   triggered #DB. A kernel #DB must leave it alone to ensure data
   consistency for ptrace.

 - Make the bitmasking of the virtual DR6 storage correct so it does not
   lose DR_STEP.
-----BEGIN PGP SIGNATURE-----
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=Hfek
-----END PGP SIGNATURE-----