ion: Always flush using kernel virtual address

Cache operations via userspace address always has a risk of a
page fault. On arm64, userspace addresses are marked as write
protect and fixed up on first accesses. If a userspace address
that has not been written to is invalidated, we will take a
fault, fixup and proceed on. If the mmap semaphore is locked at
the time however we cannot fixup the fault and will abort instead.
Rather than hope the mmap semaphore is not write protected, always
invalidate via the kernel virtual addresses instead.

Change-Id: I9f9f21a7618eb6fc74fc4438cc920d857664c428