Improvements for hw_breakpoint_handler()

Hi Frederic,
Can you please pull the second version of the patch which perform
the following.

Patch 1/2: Clears the arch-reserved bits from debug status register. This helps easy bitwise operations - such as the check for non-trap bits in hw_breakpoint_handler. A check for the same using
"if (dr6 & (~DR_TRAP_BITS))" throws incorrect results due to the presence of preset reserved bits.

Patch 2/2: Return NOTIFY_DONE from hw_breakpoint_handler() for interested user-space breakpoints (and not unconditionally) since this could lead to further processing in do_debug() with undesirable consequences.

Changelog ver II
----------------
Version I: LKML ref: [email protected]

- Return NOTIFY_DONE for further processing (i.e. signal delivery) only for interested user-space breakpoints i.e. with current->thread.debugreg6 bits set.

Thanks,
K.Prasad

Ok, looks good. I'm queueing these.

Thanks Prasad!