misc: fastrpc: Fix double free of 'buf' in error path

smatch warning:
drivers/misc/fastrpc.c:1926 fastrpc_req_mmap() error: double free of 'buf'

In fastrpc_req_mmap() error path, the fastrpc buffer is freed in
fastrpc_req_munmap_impl() if unmap is successful.

But in the end, there is an unconditional call to fastrpc_buf_free().
So the above case triggers the double free of fastrpc buf.

Fixes: 72fa6f7820c4 ("misc: fastrpc: Rework fastrpc_req_munmap")
Reviewed-by: Shuah Khan <[e-mail]>
Reviewed-by: Dan Carpenter <[e-mail]>
Reviewed-by: Srinivas Kandagatla <[e-mail]>
Signed-off-by: Sukrut Bellary <[e-mail]>
Signed-off-by: Srinivas Kandagatla <[e-mail]>
Cc: stable <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/fastrpc.c

···
 				      &args[0]);
 	if (err) {
 		dev_err(dev, "mmap error (len 0x%08llx)\n", buf->size);
-		goto err_invoke;
+		fastrpc_buf_free(buf);
+		return err;
 	}
 
 	/* update the buffer to be able to deallocate the memory on the DSP */
···
 
 err_assign:
 	fastrpc_req_munmap_impl(fl, buf);
-err_invoke:
-	fastrpc_buf_free(buf);
 
 	return err;
 }