READ_ONCE: Fix comment describing 2x32-bit atomicity

READ_ONCE: Fix comment describing 2x32-bit atomicity

READ_ONCE() permits 64-bit accesses on 32-bit architectures, since this
crops up in a few places and is generally harmless because either the
upper bits are always zero (e.g. for a virtual address or 32-bit time_t)
or the architecture provides 64-bit atomicity anyway.

Update the corresponding comment above compiletime_assert_rwonce_type(),
which incorrectly states that 32-bit x86 provides 64-bit atomicity, and
instead reference 32-bit Armv7 with LPAE.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Will Deacon <will@kernel.org>