erofs: use meta buffers for erofs_read_superblock()

The only change is that, meta buffers read cache page without __GFP_FS
flag, which shall not matter.

Link: https://lore.kernel.org/r/20220209060108.43051-7-jefflexu@linux.alibaba.com


Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>