fs: add ksys_sync() helper; remove in-kernel calls to sys_sync()

Using this helper allows us to avoid the in-kernel calls to the
sys_sync() syscall. The ksys_ prefix denotes that this function
is meant as a drop-in replacement for the syscall. In particular, it
uses the same calling convention as sys_sync().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/sparc/kernel/setup_32.c

@@ -86,7 +86,7 @@ static void prom_sync_me(void)
 	show_free_areas(0, NULL);
 	if (!is_idle_task(current)) {
 		local_irq_enable();
-		sys_sync();
+		ksys_sync();
 		local_irq_disable();
 	}
 	prom_printf("Returning to prom\n");