ANDROID: staging: ion: move buffer kmap from begin/end_cpu_access()

Since dma_buf_begin/end_cpu_access() calls always used to bracket
dma_buf_kmap/kunmap calls, ION performed kmap/kunmap invocations for the
buffer during dma_buf_begin/end_cpu_access() calls and cached the
results with a kmap counter.
However, dma_buf_begin/end_cpu_access() invocations can be
triggered from the userspace using the DMA_BUF_IOC_SYNC ioctl as well.
This means that a mapping that was created by a device driver using by a
dma_buf_kmap() call or an ion_map_kernel() call could be unmapped
from userspace if a client accidentally(or maliciously) invoked
DMA_BUF_IOCTL_SYNC IOCTL with 'DMA_BUF_SYNC_END' argument since this
would inturn invoke dma_buf_end_cpu_access() which would then decrement
the kmap counter and invoke kunmap() when the counter gets to 0.

This patch moves the kmap/kunmap operations from the
begin/end_cpu_access() DMA-BUF ops to the map/unmap DMA-BUF ops to
prevent the issue.

Bug: 187527909
Change-Id: I00dc8eefefb1f3aab99e770f90d624011f7740f0
[hridya: minor conflicts during cherry-picking]
Signed-off-by: Hridya Valsaraju <hridya@google.com>
Issue: FP3SEC-110
(cherry picked from commit 41a097c0)
(cherry picked from commit 4673c6b7)
(cherry picked from commit e885419b0c8b8fb3a1f22cbe9df097f95a748cb5)