internal/poll, net, os: remove poll.Splice syscall name return value

internal/poll, net, os: remove poll.Splice syscall name return value

The sc return value of internal/poll.Splice is always set to the same
value "splice" in the error case and then passed to wrapSyscallError.
Move that value to the wrapSyscallError calls to simplify the code a
bit.

Change-Id: I98104d755da68ff9f301fabc43c2618fda21a175
Reviewed-on: https://go-review.googlesource.com/c/go/+/575655
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>