Use `z_idata_size != 0` to indicate that ztailpacking is enabled.
`Z_EROFS_ADVISE_INLINE_PCLUSTER` cannot be ignored, as `h_idata_size`
could be non-zero prior to erofs-utils 1.6 [1].
Additionally, merge `z_idataoff` and `z_fragmentoff` since these two
features are mutually exclusive for a given inode.
[1] https://git.kernel.org/xiang/erofs-utils/c/547bea3cb71a
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>