erofs: clean up `enum z_erofs_collectmode'

`enum z_erofs_collectmode' is really ambiguous, but I'm not quite
sure if there are better naming, basically it's used to judge whether
inplace I/O can be used due to the current status of pclusters in
the chain.

Rename it as `enum z_erofs_pclustermode' instead.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>