powerpc: Enable sys_kcmp() for CRIU

The commit 8170a83f ("powerpc: Wireup the kcmp syscall to sys_ni") has
disabled the kcmp syscall for powerpc.  This has been done due to the use
of unsigned long parameters which may require a dedicated wrapper to handle
32bit process on top of 64bit kernel.  However in the kcmp() case, the 2
unsigned long parameters are currently only used to carry file descriptors
from user space to the kernel.  Since such a parameter is passed through
register, and file descriptor doesn't need to get extended, there is,
today, no need for a wrapper.

In the case there will be a need to pass address in or out of this system
call, then a wrapper could be required, it will then be to care of it.

As today this is not the case, it is safe to enable kcmp() on powerpc.

Tested (by Laurent) on 64-bit, 32-bit, and 32-bit userspace on 64-bit
kernel using tools/testing/selftests/kcmp [mpe].
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/systbl.h

@@ -358,7 +358,7 @@ SYSCALL_SPU(setns)
 COMPAT_SYS(process_vm_readv)
 COMPAT_SYS(process_vm_writev)
 SYSCALL(finit_module)
-SYSCALL(ni_syscall) /* sys_kcmp */
+SYSCALL(kcmp) /* sys_kcmp */
 SYSCALL_SPU(sched_setattr)
 SYSCALL_SPU(sched_getattr)
 SYSCALL_SPU(renameat2)