* fhandler.cc (fhandler_base_overlapped::raw_write): When performing
nonblocking I/O, copy user space data into own buffer. Add longish
comment to explain why.
* fhandler.h (fhandler_base_overlapped::atomic_write_buf): New member.
(fhandler_base_overlapped::fhandler_base_overlapped): Initialize
atomic_write_buf.
(fhandler_base_overlapped::fhandler_base_overlapped): New destructor,
free'ing atomic_write_buf.
(fhandler_base_overlapped::copyto): Set atomic_write_buf to NULL in
copied fhandler.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>