erofs: simplify tail inline pcluster handling

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


Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250225114038.3259726-1-hsiangkao@linux.alibaba.com