internal/poll: raise Linux's maxCopyFileRangeRound to MAX_RW_COUNT

Change-Id: I23c0c850ba57e7a49b78159d9293d1d25e1d2340
Reviewed-on: https://go-review.googlesource.com/c/go/+/606637


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>