vfs: move open right after ->tmpfile()

Create a helper finish_open_simple() that opens the file with the original
dentry.  Handle the error case here as well to simplify callers.

Call this helper right after ->tmpfile() is called.

Next patch will change the tmpfile API and move this call into tmpfile
instances.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>