Refactor out the sendfile copyout in order to make vn_sendfile()
callable from the kernel.

Right now vn_sendfile() can't be called from anything other than
a syscall handler _and_ return the number of bytes queued.
This simply moves the copyout() to do_sendfile() so that any kernel
code can initiate vn_sendfile() outside of a syscall context.

Tested:

* tiny little sendfile program spitting things out a tcp socket

Sponsored by:	Netflix, Inc.