new_sync_write(): discard ->ki_pos unless the return value is positive

That allows ->write_iter() instances much more convenient life wrt
iocb->ki_pos (and fixes several filesystems with borderline POSIX
violations when zero-length write succeeds and changes the current
position).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/read_write.c
Simple merge