erofs: convert erofs_read_metabuf() to erofs_bread() for xattr

buf->inode is constant once initialized during erofs_buf's lifetime.
Thus call erofs_init_metabuf() and erofs_bread() separately to avoid
the repetition of assigning buf->inode when iterating xattrs.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230601024347.108469-2-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>