Jiankang reports that our race detection in set_pte_at is firing when
copying the page tables in dup_mmap as a result of a fork(). In this
situation, the page table isn't actually live and so there is no way
that we can race with a concurrent update from the hardware page table
walker.

This patch reworks the race detection so that we require either the
mm to match the current active_mm (i.e. currently installed in our TTBR0)
or the mm_users count to be greater than 1, implying that the page table
could be live in another CPU. The mm_users check might still be racy,
but we'll avoid false positives and it's not realistic to validate that
all the necessary locks are held as part of this assertion.

Cc: Yisheng Xie <xieyisheng1@huawei.com>
Reported-by: NJiankang Chen <chenjiankang1@huawei.com>
Tested-by: NJiankang Chen <chenjiankang1@huawei.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>