net: update file read position after sendfile syscall

net: update file read position after sendfile syscall

On dragonfly, freebsd and solaris the sendfile syscall does not update
the read position of the source fd. Update it after sendfile so
successive calls start at the correct position.

Fixes #25809

Change-Id: Iaac79f89704b75b8038d4bb60eaf793a262cdd8f
Reviewed-on: https://go-review.googlesource.com/117895
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>