powerpc: silence a -Wcast-function-type warning in dawr_write_file_bool

In commit c1fe190c ("powerpc: Add force enable of DAWR on P9
option") the following piece of code was added:

   smp_call_function((smp_call_func_t)set_dawr, &null_brk, 0);

Since GCC 8 this triggers the following warning about incompatible
function types:

  arch/powerpc/kernel/hw_breakpoint.c:408:21: error: cast between incompatible function types from 'int (*)(struct arch_hw_breakpoint *)' to 'void (*)(void *)' [-Werror=cast-function-type]

Since the warning is there for a reason, and should not be hidden behind
a cast, provide an intermediate callback function to avoid the warning.

Fixes: c1fe190c ("powerpc: Add force enable of DAWR on P9 option")
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/hw_breakpoint.c

@@ -384,6 +384,11 @@ void hw_breakpoint_pmu_read(struct perf_event *bp)
 bool dawr_force_enable;
 EXPORT_SYMBOL_GPL(dawr_force_enable);
 
+static void set_dawr_cb(void *info)
+{
+	set_dawr(info);
+}
+
 static ssize_t dawr_write_file_bool(struct file *file,
 				    const char __user *user_buf,
 				    size_t count, loff_t *ppos)
@@ -403,7 +408,7 @@ static ssize_t dawr_write_file_bool(struct file *file,
 
 	/* If we are clearing, make sure all CPUs have the DAWR cleared */
 	if (!dawr_force_enable)
-		smp_call_function((smp_call_func_t)set_dawr, &null_brk, 0);
+		smp_call_function(set_dawr_cb, &null_brk, 0);
 
 	return rc;
 }