orangefs: fix double-free of trailer_buf on readdir copy failure

In the Linux kernel, the following vulnerability has been resolved: orangefs: fix double-free of trailer_buf on readdir copy failure On a readdir downcall, orangefs_devreq_write_iter() frees op->downcall.trailer_buf with vfree() when copy_from_iter_full() fails, but does not clear the pointer before goto Efault. The waiter in do_readdir() is then woken with a negative status and frees the same pointer again on its r < 0 path, causing a deterministic double-free. A client holding /dev/pvfs2-req triggers it by sending a readdir downcall whose declared trailer_size exceeds the bytes it supplies. Clear the pointer after freeing so the readdir-side vfree() becomes a no-op.