hw-breakpoints: Zeroe the breakpoint attrs on initialization

The perf attrs used to set up breakpoint parameters are often allocated
in the stack and not zeroed out before calling hw_breakpoint_init().
Handle it from this helper to avoid random attributes set by the stack.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Prasad <prasad@linux.vnet.ibm.com>