x86/asm/entry/32: Reinstate clearing of pt_regs->r8..r11 on EFAULT path

I broke this recently when I changed pt_regs->r8..r11 clearing
logic in INT 80 code path.

There is a branch from SYSENTER/SYSCALL code to INT 80 code:
if we fail to retrieve arg6, we return EFAULT. Before this
patch, in this case we don't clear pt_regs->r8..r11.

This patch fixes this. The resulting code is smaller and
simpler.

While at it, remove incorrect comment about syscall dispatching
CALL insn: it does not use RIP-relative addressing form (the
comment was meant to be "TODO: make this rip-relative", and
morphed since then, dropping "TODO").

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1433701470-28800-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64_compat.S

@@ -413,9 +413,7 @@ END(entry_SYSCALL_compat)
 
 ia32_badarg:
 	ASM_CLAC
-	movq	$-EFAULT, %rax
-	jmp	ia32_sysret
-
+	movq	$-EFAULT, RAX(%rsp)
 ia32_ret_from_sys_call:
 	xorl	%eax, %eax		/* Do not leak kernel information */
 	movq	%rax, R11(%rsp)
@@ -486,9 +484,7 @@ ia32_do_call:
 	cmpq	$(IA32_NR_syscalls-1), %rax
 	ja	1f
 
-	call	*ia32_sys_call_table(, %rax, 8) /* RIP relative */
-
-ia32_sysret:
+	call	*ia32_sys_call_table(, %rax, 8)
 	movq	%rax, RAX(%rsp)
 1:
 	jmp	int_ret_from_sys_call