staging: erofs: fix Warning Use BUG_ON instead of if condition followed by BUG

fix below warning reported by  coccicheck

drivers/staging/erofs/unzip_pagevec.h:74:2-5: WARNING: Use BUG_ON
instead of if condition followed by BUG.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
[ Gao Xiang: use DBG_BUGON instead of BUG_ON for eng version only. ]
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>