[review request] make sendfile usable from inside the kernel itself

[Adrian Chadd]

Hi,

http://people.freebsd.org/~adrian/netflix/20131125-sendfile-shuffle-1.diff

This moves the copyout to the 'sbytes' field occur _outside_ of
vn_sendfile() so vn_sendfile() itself can be called from kernel code.

There's likely some more shuffling I have to do with sendfile but this
seems like a good, simple starting point.

I've done some light testing with some local code to no ill effect.

Thanks!



-adrian