internal/poll: drop redundant ENOSYS in CopyFileRange
Update CL 425881 and CL 428396
I browsed the source code related to copy_file_range in the kernel and found that the latest kernel may still return EXDEV errors in copy_file_range(2) due to certain cases, for details see: https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1559, https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1479, and
https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1439.
Therefore, the EXDEV still needs to be kept, but the ENOSYS error can be safely removed.
Change-Id: I47026b8dd33f7ffc4de1306af6b67c7b4d2062d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/428555
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Reviewed-by: Ian Lance Taylor <iant@google.com>
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