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 43a43faf5376 ("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 <[email protected]>
Cc: Christophe Leroy <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Linus Torvalds <[email protected]>

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