x86/syscalls/64: Add compat_sys_keyctl for 32-bit userspace

x86_64 needs to use compat_sys_keyctl for 32-bit userspace rather than
calling sys_keyctl(). The latter will work in a lot of cases, thereby
hiding the issue.

Reported-by: Stephan Mueller <smueller@chronox.de>
Tested-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: keyrings@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/146961615805.14395.5581949237156769439.stgit@warthog.procyon.org.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/syscalls/syscall_32.tbl

@@ -294,7 +294,7 @@
 # 285 sys_setaltroot
 286	i386	add_key			sys_add_key
 287	i386	request_key		sys_request_key
-288	i386	keyctl			sys_keyctl
+288	i386	keyctl			sys_keyctl			compat_sys_keyctl
 289	i386	ioprio_set		sys_ioprio_set
 290	i386	ioprio_get		sys_ioprio_get
 291	i386	inotify_init		sys_inotify_init