module/decompress: Fix error checking on zstd decompression

While implementing support for in-kernel decompression in kmod,
finit_module() was returning a very suspicious value:

	finit_module(3, "", MODULE_INIT_COMPRESSED_FILE) = 18446744072717407296

It turns out the check for module_get_next_page() failing is wrong,
and hence the decompression was not really taking place. Invert
the condition to fix it.

Fixes: 169a58a ("module/decompress: Support zstd in-kernel decompression")
Cc: stable@kernel.org
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

Author: Lucas De Marchi

kernel/module/decompress.c

@@ -257,7 +257,7 @@ static ssize_t module_zstd_decompress(struct load_info *info,
 	do {
 		struct page *page = module_get_next_page(info);
 
-		if (!IS_ERR(page)) {
+		if (IS_ERR(page)) {
 			retval = PTR_ERR(page);
 			goto out;
 		}