erofs: unify xattr_iter structures

Unify xattr_iter/listxattr_iter/getxattr_iter structures into
erofs_xattr_iter structure.

This is in preparation for the following further cleanup.
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613074114.120115-3-jefflexu@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>