After #4906, containerd opens fifo in read/write mode in linux platform
The original comment doesn't correct and is removed by #5174.
```
// original comment
// When using a multi-container shim, the fifo of the 2nd to Nth
// container will not be opened when the ctx is done. This will
// cause an ErrReadClosed that can be ignored.
```
However, we should add comment for checkCopyShimLogError to mention why
we call checkCopyShimLogError. The checkCopyShimLogError, it is to prevent
the flood of expected error messages after task die and the expected
errors depend on platform.
Signed-off-by: Wei Fu <fuweid89@gmail.com>
9fdc96c095