erofs: use meta buffers for xattr operations

Get rid of old erofs_get_meta_page() within xattr operations by
using on-stack meta buffers in order to prepare subpage and folio
features.

Link: https://lore.kernel.org/r/20220102040017.51352-5-hsiangkao@linux.alibaba.com


Reviewed-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>