internal/poll: don't use r/w lock for Pread/Pwrite

internal/poll: don't use r/w lock for Pread/Pwrite

Since Pread/Pwrite specify a file offset, using incref is sufficient.
This permits multiple Pread/Pwrite calls in parallel.

Since Pread/Pwrite specify a file offset, it doesn't seem to make
sense to use the poller for them, so don't.

Updates #19586

Change-Id: I676be16bf519b9a45f8e6b1d991c44f10848bc11
Reviewed-on: https://go-review.googlesource.com/41670
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>