x86/entry/compat: Keep TS_COMPAT set during signal delivery

x86/entry/compat: Keep TS_COMPAT set during signal delivery

Signal delivery needs to know the sign of an interrupted syscall's
return value in order to detect -ERESTART variants.  Normally this
works independently of bitness because syscalls internally return
long.  Under ptrace, however, this can break, and syscall_get_error
is supposed to sign-extend regs->ax if needed.

We were clearing TS_COMPAT too early, though, and this prevented
sign extension, which subtly broke syscall restart under ptrace.

Reported-by: Robert O'Callahan <robert@ocallahan.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
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: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org # 4.3.x-
Fixes: c5c46f59e4e7 ("x86/entry: Add new, comprehensible entry and exit handlers written in C")
Link: http://lkml.kernel.org/r/cbce3cf545522f64eb37f5478cb59746230db3b5.1455142412.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>