dma-buf: fix UAF in dma_buf_fd() tracepoint

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

dma-buf: fix UAF in dma_buf_fd() tracepoint

Once FD_ADD() returns, the fd is live in the file descriptor table

and a thread sharing that table can close() it before DMA_BUF_TRACE()

runs. The close drops the last reference, __fput() frees the dma_buf,

and the tracepoint then dereferences dmabuf to take dmabuf->name_lock

-- slab-use-after-free.

Split FD_ADD() back into get_unused_fd_flags() + fd_install() and

emit the tracepoint between them. While the fdtable slot is reserved

with a NULL file pointer, a racing close() returns -EBADF without

entering __fput(), so the dma_buf stays alive across the trace. Same

approach as commit 2d76319c4cbb ("dma-buf: fix UAF in dma_buf_put()

tracepoint").

This undoes the FD_ADD() conversion done in commit 34dfce523c90

("dma: convert dma_buf_fd() to FD_ADD()"); FD_ADD() has no place to

hook the tracepoint safely.