usb: gadget: f_fs: Use struct_size() and flex_array_size() helpers

Make use of the struct_size() and flex_array_size() helpers instead of
an open-coded version, in order to avoid any potential type mistakes
or integer overflows that, in the worst scenario, could lead to heap
overflows.

Also, address the following sparse warnings:
drivers/usb/gadget/function/f_fs.c:922:23: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20220120222933.GA35155@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -919,12 +919,12 @@ static ssize_t __ffs_epfile_read_data(struct ffs_epfile *epfile,
 		data_len, ret);
 
 	data_len -= ret;
-	buf = kmalloc(sizeof(*buf) + data_len, GFP_KERNEL);
+	buf = kmalloc(struct_size(buf, storage, data_len), GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 	buf->length = data_len;
 	buf->data = buf->storage;
-	memcpy(buf->storage, data + ret, data_len);
+	memcpy(buf->storage, data + ret, flex_array_size(buf, storage, data_len));
 
 	/*
 	 * At this point read_buffer is NULL or READ_BUFFER_DROP (if