xen: fix UAF in dmabuf_exp_from_pages()

In the Linux kernel, the following vulnerability has been resolved:

xen: fix UAF in dmabuf_exp_from_pages()

[dma_buf_fd() fixes; no preferences regarding the tree it goes through - up to xen folks]

As soon as we'd inserted a file reference into descriptor table, another thread could close it. That's fine for the case when all we are doing is returning that descriptor to userland (it's a race, but it's a userland race and there's nothing the kernel can do about it). However, if we follow fd_install() with any kind of access to objects that would be destroyed on close (be it the struct file itself or anything destroyed by its ->release()), we have a UAF.

dma_buf_fd() is a combination of reserving a descriptor and fd_install(). gntdev dmabuf_exp_from_pages() calls it and then proceeds to access the objects destroyed on close - starting with gntdev_dmabuf itself.

Fix that by doing reserving descriptor before anything else and do fd_install() only when everything had been set up.