[PATCH] erofs: fix missing folio_unlock causing lock imbalance

[Zhan Xusheng]

Hi Gao,

Thanks for the detailed explanation.

I see your point that folios attached to a pcluster should
always be managed by EROFS, and the !erofs_folio_is_managed()
case should not happen in practice.

Instead of handling it silently, it makes more sense to
treat this as an invariant and catch unexpected cases
explicitly.

I'll update the patch accordingly by adding DBG_BUGON()
and send a v2.

Thanks,
Zhan Xusheng