habanalabs: fix UAF in export_dmabuf()

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().
habanalabs export_dmabuf() calls it and then proceeds to access the
objects destroyed on close.  In particular, it grabs an extra reference to
another struct file that will be dropped as part of ->release() for ours;
that "will be" is actually "might have already been".

Fix that by reserving descriptor before anything else and do fd_install()
only when everything had been set up.  As a side benefit, we no longer
have the failure exit with file already created, but reference to
underlying file (as well as ->dmabuf_export_cnt, etc.) not grabbed yet;
unlike dma_buf_fd(), fd_install() can't fail.

Fixes: db1a8dd9 ("habanalabs: add support for dma-buf exporter")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>