UPSTREAM: psi: Fix a division error in psi poll()

The psi window size is a u64 an can be up to 10 seconds right now,
which exceeds the lower 32 bits of the variable. We currently use
div_u64 for it, which is meant only for 32-bit divisors. The result is
garbage pressure sampling values and even potential div0 crashes.

Use div64_u64.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Jingfeng Xie <xiejingfeng@linux.alibaba.com>
Link: https://lkml.kernel.org/r/20191203183524.41378-3-hannes@cmpxchg.org


Signed-off-by: Sasha Levin <sashal@kernel.org>

(cherry picked from commit c3466952ca1514158d7c16c9cfc48c27d5c5dc0f)

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I49fdfd55751d1a2cde19666624c9c5d76dc78dad