staging: erofs: unzip_{pagevec.h,vle.c}: rectify BUG_ONs

commit 70b17991d89554cdd16f3e4fb0179bcc03c808d9 upstream.

remove all redundant BUG_ONs, and turn the rest
useful usages to DBG_BUGONs.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>