powerpc/hw_breakpoint: Only disable hw breakpoint if cpu supports it

We get the below warning if we try to use kexec on P9:
   kexec_core: Starting new kernel
   WARNING: CPU: 0 PID: 1223 at arch/powerpc/kernel/process.c:826 __set_breakpoint+0xb4/0x140
   [snip]
   NIP __set_breakpoint+0xb4/0x140
   LR  kexec_prepare_cpus_wait+0x58/0x150
   Call Trace:
     0xc0000000ee70fb20 (unreliable)
     0xc0000000ee70fb20
     default_machine_kexec+0x234/0x2c0
     machine_kexec+0x84/0x90
     kernel_kexec+0xd8/0xe0
     SyS_reboot+0x214/0x2c0
     system_call+0x58/0x6c

This happens since we are trying to clear hw breakpoint on POWER9,
though we don't have CPU_FTR_DAWR enabled. Guard __set_breakpoint()
within hw_breakpoint_disable() with ppc_breakpoint_available() to
address this.

Fixes: 96541531 ("powerpc: Disable DAWR in the base POWER9 CPU features")
Signed-off-by: NNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/hw_breakpoint.h

@@ -80,7 +80,8 @@ static inline void hw_breakpoint_disable(void)
 	brk.address = 0;
 	brk.type = 0;
 	brk.len = 0;
-	__set_breakpoint(&brk);
+	if (ppc_breakpoint_available())
+		__set_breakpoint(&brk);
 }
 extern void thread_change_pc(struct task_struct *tsk, struct pt_regs *regs);
 int hw_breakpoint_handler(struct die_args *args);