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.

Fixes: a240d6e4 ("xen/gntdev: Implement dma-buf export functionality")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250712050916.GY1880847@ZenIV>
Signed-off-by: Juergen Gross <jgross@suse.com>