ovl: keep err zero after successful ovl_cache_get()

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

ovl: keep err zero after successful ovl_cache_get()

ovl_iterate_merged() stores PTR_ERR(cache) in err before checking

IS_ERR(cache). On success err holds the truncated cache pointer and

can be returned as a bogus non-zero error.

The syzbot reproducer reaches this through overlay-on-overlay readdir:

getdents64

iterate_dir(outer overlay file)

ovl_iterate_merged()

ovl_cache_get()

ovl_dir_read_merged()

ovl_dir_read()

iterate_dir(inner overlay file)

ovl_iterate_merged()

Only compute PTR_ERR(cache) on the error path.