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@xxxxxxxxxxxxxxxxx>