powerpc: avoid discarding flags in system_call_exception()

Some thread flags can be set remotely, and so even when IRQs are
disabled, the flags can change under our feet. Thus, when setting flags
we must use an atomic operation rather than a plain read-modify-write
sequence, as a plain read-modify-write may discard flags which are
concurrently set by a remote thread, e.g.

	// task A			// task B
	tmp = A->thread_info.flags;
					set_tsk_thread_flag(A, NEWFLAG_B);
	tmp |= NEWFLAG_A;
	A->thread_info.flags = tmp;

In arch/powerpc/kernel/interrupt.c's system_call_exception(), we set
_TIF_RESTOREALL in the thread info flags with a read-modify-write, which
may result in other flags being discarded.

Elsewhere in the file we use clear_bits() to atomically remove flag
bits, so let's use set_bits() here for consistency with those.

I presume there may be reasons (e.g. instrumentation) that prevent the
use of set_thread_flag() and clear_thread_flag() here, which would
otherwise be preferable.

Fixes: ae7aaec ("powerpc/64s: system call rfscv workaround for TM bugs")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Eirik Fuller <efuller@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>

arch/powerpc/kernel/interrupt.c

@@ -148,7 +148,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
 	 */
 	if (IS_ENABLED(CONFIG_PPC_TRANSACTIONAL_MEM) &&
 			unlikely(MSR_TM_TRANSACTIONAL(regs->msr)))
-		current_thread_info()->flags |= _TIF_RESTOREALL;
+		set_bits(_TIF_RESTOREALL, current_thread_info()->flags);
 
 	/*
 	 * If the system call was made with a transaction active, doom it and