erofs: use absolute position in xattr iterator

erofs: use absolute position in xattr iterator

Replace blkaddr/ofs with pos in 'struct erofs_xattr_iter'.

After erofs_bread() is introduced to replace raw page cache APIs for
metadata I/Os handling, xattr_iter_fixup() is no longer needed anymore.

In addition, it is also unnecessary to check if the iterated position is
span over the block boundary as absolute offset is used instead of
blkaddr + offset pairs.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613074114.120115-2-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>