futex: fix handling of read-only-mapped hugepages

The hugepage code had the exact same bug that regular pages had in
commit 7485d0d3758e ("futexes: Remove rw parameter from
get_futex_key()").

The regular page case was fixed by commit 9ea71503a8ed ("futex: Fix
regression with read only mappings"), but the transparent hugepage case
(added in a5b338f2b0b1: "thp: update futex compound knowledge") case
remained broken.

Found by Dave Jones and his trinity tool.

Reported-and-tested-by: Dave Jones <davej@fedoraproject.org>
Cc: stable@kernel.org # v2.6.38+
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
futex: move user address verification up to common code

When debugging the read-only hugepage case, I was confused by the fact
that get_futex_key() did an access_ok() only for the non-shared futex
case, since the user address checking really isn't in any way specific
to the private key handling.

Now, it turns out that the shared key handling does effectively do the
equivalent checks inside get_user_pages_fast() (it doesn't actually
check the address range on x86, but does check the page protections for
being a user page).  So it wasn't actually a bug, but the fact that we
treat the address differently for private and shared futexes threw me
for a loop.

Just move the check up, so that it gets done for both cases.  Also, use
the 'rw' parameter for the type, even if it doesn't actually matter any
more (it's a historical artifact of the old racy i386 "page faults from
kernel space don't check write protections").

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
merged futex