erofs: fix error return code in erofs_fscache_{meta_,}read_folio

If erofs_fscache_alloc_request fail and then goto out, it will return 0.
it should return a negative error code instead of 0.

Fixes: d435d532

 ("erofs: change to use asynchronous io for fscache readpage/readahead")
Signed-off-by: Sun Ke <sunke32@huawei.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220815034829.3940803-1-sunke32@huawei.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>