futex: fix user access on powerpc

The powerpc user access code is special, and unlike other architectures
distinguishes between user access for reading and writing.

And commit 43a43fa ("futex: improve user space accesses") messed
that up.  It went undetected elsewhere, but caused ppc32 to fail early
during boot, because the user access had been started with
user_read_access_begin(), but then finished off with just a plain
"user_access_end()".

Note that the address-masking user access helpers don't even have that
read-vs-write distinction, so if powerpc ever wants to do address
masking tricks, we'll have to do some extra work for it.

[ Make sure to also do it for the EFAULT case, as pointed out by
  Christophe Leroy ]

Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/all/87bjxl6b0i.fsf@igel.home/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: torvalds

kernel/futex/futex.h

@@ -265,11 +265,11 @@ static __always_inline int futex_read_inatomic(u32 *dest, u32 __user *from)
 	else if (!user_read_access_begin(from, sizeof(*from)))
 		return -EFAULT;
 	unsafe_get_user(val, from, Efault);
-	user_access_end();
+	user_read_access_end();
 	*dest = val;
 	return 0;
 Efault:
-	user_access_end();
+	user_read_access_end();
 	return -EFAULT;
 }