internal/poll: use copy_file_range only on Linux kernel >= 5.3

https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS states:

  A major rework of the kernel implementation occurred in 5.3.  Areas
  of the API that weren't clearly defined were clarified and the API
  bounds are much more strictly checked than on earlier kernels.
  Applications should target the behaviour and requirements of 5.3
  kernels.

Rather than attempting to detect the file system for source and
destination files (which means two additional statfs syscalls) and skip
copy_file_range in case of known defects (e.g. CIFS -> CIFS), just
assume copy_file_range to be broken on kernels < 5.3.

Fixes #42400

Change-Id: I3a531296182c1d6e341772cc9d2be5bf83e52575
Reviewed-on: https://go-review.googlesource.com/c/go/+/268338


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>