Simplify the logic in z_erofs_fill_inode_lazy() by combining the
handling of ztailpacking and fragments, as they are mutually exclusive.
Note that `h->h_clusterbits >> Z_EROFS_FRAGMENT_INODE_BIT` is handled
above, so no need to duplicate the check.
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>