HW-BKPT: Allow per-cpu kernel-space Hardware Breakpoint requests

Hi All,
Please find a patch that enables kernel-space breakpoints to be
requested for a subset of the available CPUs in the system. This allows per-CPU breakpoints and comes with the associated benefit of reduced overhead during (un)registration.

This enhancement allows exploitation of hardware breakpoint registers by 'perf' which produces a CPU-wise information.

Design changes
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- Every breakpoint request 'consumes' the first available debug register (starting from HBP_NUM) in each CPU represented by 'cpumask' field in 'struct hw_breakpoint'.

- 'hbp_kernel_pos' (that separates kernel-space breakpoints from the free/user-space breakpoints) now points to the maximum of debug registers consumed on any given CPU.
-- 'hbp_kernel_pos' is decremented (one-at-a-time) to allow a new-slot
for kernel-space requests iff all debug registers on the given CPU (from HBP_NUM - 1 to 'hbp_kernel_pos' are already consumed.
-- 'hbp_kernel_pos' is incremented (one-at-a-time) to free a slot iff
a removal request results in the release of a bkpt request that consumed maximum debug registers for kernel-space.

- Every removal request results in compaction of breakpoint registers (on a per-cpu basis) to occupy the vacant debug register.

The patch is based on commit b6c720b811aed0eeda89f277f13c1bd1bdf721fd of -tip tree and has been tested to work fine on an x86 machine for both cases (i.e. system-wide kernel breakpoints and bkpts for a subset of CPUs).

Please let me know your comments on the same.

Thanks,
K.Prasad