erofs: ensure all folios are managed in erofs_try_to_free_all_cached_folios()

folio_trylock() in erofs_try_to_free_all_cached_folios() may
successfully acquire the folio lock, but the subsequent check
for erofs_folio_is_managed() can skip unlocking when the folio
is not managed by EROFS.

As Gao Xiang pointed out, this condition should not happen in
practice because compressed_bvecs[] only holds valid cached folios
at this point — any non-managed folio would have already been
detached by z_erofs_cache_release_folio() under folio lock.

Fix this by adding DBG_BUGON() to catch unexpected folios
and ensure folio_unlock() is always called.

Suggested-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chunhai Guo <guochunhai@vivo.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>